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KLB
03-30-2003, 09:47 PM
I recently received two copper caps, a WZ34 and a W55/175 (supposed to be a 55v drop at 175ma current draw.) I am dissapointed in them. Both have more sag than specified when compared with a Mullard GZ34.

The amp is a Deluxe Reverb with GZ34, Dave Allen's oversized PT/OT/Choke set (180ma current!) and JJ 6L6GC. Initially, using the GZ34, I set the bias at 41ma and 428v for 17.5w idle dissipation using the transformer shunt method. I measured a few things, then swapped in the WZ34 copper cap without rebiasing:

Standby/Plate/V1, pin 1
GZ34 - 477v/428v/201v
WZ34 - 475v/401v

After installing the WZ34, the idle dissipation worked out to be less than 12w. This was my first sign that the Weber Copper Cap may not be a direct replacement for a tube rectifier. I rebiased with the WZ34 for 17.5w idle dissipation. The new numbers are:

46.5 ma @ 376v plate, 176v on V1, pin #1.

As you see, the WZ34 has an additional 52v of sag compared to the WZ34 under the same idle load condition. While this may be a musically useful outcome for certain playing styles, it does not duplicate the performance of a GZ34.

Out of curiosity, I swapped in the W55/175 and attempted to bias for an idle dissipation of 17.5w. I couldn't quite get there due to the values of resistor in the bias circuit.

Here's what I found: 43.5 ma @ 363v = 15.6w, 169v on V1, pin #1. This is 13v more sag than the WZ34 with 1.9w lower idle dissipation. Since I can't make a direct comparison with a 17.5w bias setting, I am guessing the W55/175 has a additional 20+v of sag compared to the WZ34 sample I have. This puts it in the neighborhood of 90 volts (17v normal GZ34 drop + 52v WZ34 + 22v W55175) total sag vs. simple diode rectification! And this is with a load that is less than 175ma -- its rated current draw for a 55v sag.

These are not the results I expected. The copper cap remains a good idea, but I don't know how to predict the results based on Weber's model descriptions. I suppose through trial and error one could find a version that gives a desired drop in plate voltage and current sag.

Perhaps the units I received do not have the correct internal parts to match the specification. I e-mailed the above information to WeberVST. Ted's very prompt response was:

"We sell a ton of those with very few complaints. Bruce Collins at Mission Amps assisted me in determining the original values. I also used the nominal drop values as specified in the various tube manuals. Using the official specs from the tube manuals, it is simple ohms law. There is nothing dynamic about the forward transfer function of a tube rectifier. Some GZ34's may actually be better (stiffer) than the nominal specs.

Send them back and we will refund your money."

There you have it. I gave Weber some objective data that indicates their "simple ohms law" formula doesn't match the real world. They weren't at all concered that both units might not be in spec. Oh well, at least I can get a refund. I had high hopes for their use...

Ken

PS - The new Allen OT/PT set for the DR kicks booty! See my separate post on that.

John Phillips
03-31-2003, 03:05 AM
Interesting. This is the second time I've heard this (unless you are Joe Rush from another forum).

It is curious. He's right in theory - from the tube spec, you can calculate the simple resistance needed to achieve the same effect. But perhaps the tube spec does not match reality. The Copper Caps may be in spec (for how they were designed), but that's not necessarily the same thing.

Are your current measurements total draw, or just the power tubes? I'm assuming you've measured the total (including the preamp draw and any through bleeder resistors), but if so it is strange that a unit specified for a particular voltage drop at a particular current (the W55/175) does not give those results. I would have thought it was a simple case of working out the resistance value need to give that drop (314 ohms in this case). I think that the Copper Cap contains other components designed to limit the inrush current - perhaps this is the problem, I don't know.

But I would have thought that if you measure your own GZ34, and get Weber to build one that duplicates the voltage drop at a specified current, you should have no problem.

John Phillips
03-31-2003, 05:41 AM
OK, I decided to actually measure a real Mullard GZ34. I was very surprised by what I found. I used a cathode-biased GZ34 + 2xEL34 +1x12AX7 amp I'm working on at the moment (Selmer Zodiac 30, power section only).

Unloaded secondary voltage: 555V.

Loaded secondary voltage: 543; rec output voltage 476V. Voltage drop 67V. Total B+ current 137.5mA. Rectifier resistance 67/.1375 = 487 ohms.

Since the current seemed a little high (current without power tubes is 5mA, so each tube is taking 66mA, dissipating 31W at that plate voltage - too much) I decided to rebias slightly.

Now...

Loaded secondary voltage 550; rec output voltage 490. Voltage drop 60V. Total current 110mA. Rectifier resistance 60/.11 = 545 ohms.

This achieved about the right power tube dissipation too, but that's not what's so interesting - it's that the rectifier resistance is LOWER at the higher current. This is the exact opposite of what I expected.

I checked these results with another GZ34 (unlabeled, but I'm pretty sure it's a Mullard) - the same within 1%.

I also tried (using a scrap EL34) running just one power tube. Now, voltage drop is 54V at a current of 81mA - which is why I was using a scrap tube! - but that's a rectifier resistance of 667 ohms, again confirming that the resistance falls as current increases.

This shows that actually Weber (and my own assumption) is wrong - the rectifier tube does not have a fixed resistance. But it varies in an unexpected way.

It's hard to get accurate figures from KLB's measurements without knowing the total current, but I'm going to guess 40mA for the preamp (6 tubes not 1, and one is the reverb driver), which gives, from the 477/428 (49V drop) at 41mA per tube and 40mA for the preamp gives 402 ohms @122mA for the GZ34 - quite a bit lower than what I found, and 475/401 (74V) at 30mA (? - going from "12W" at 401V) about 740 ohms @100mA for the WZ34 - much more.

My guess is that Weber has based the resistance of the WZ34 on that of a real GZ34 at a much lower current draw - about 55mA if the relationship is roughly linear.

hogy, if you're reading this, do you have any comparable figures?

PaulC
03-31-2003, 07:36 AM
John - did you check your pwr tranny secondary voltages? I've been playing around with trying to sim tube rects, and I've found that alot of sag is actually coming from the pwr tranny. Do your tests at stby/quiescence/full tilt boogie. Keep the secondary AC the same for all three and you'll notice the value of resistance for the rect is much lower when it's not factoring in the tranny voltage drop. Also alot of the non linear look to voltage sag comes from the tranny and it's ability to source current.

But I've known Ted for a few years. He is a GREAT guy. I know alot of people in the biz, and Ted and his son T.A. are about the coolest, most down to earth people I've met. We always hook up with a big group of people for dinner at the NAMM shows. The last one we did he took up the tab! One thing about posts like this - tell the man you're posting a somewhat neg review of his product. Even if you are right in everything you say the man deserves a right to see, and make any comments about the topic. Every email story has two sides. PaulC

KLB
03-31-2003, 09:01 AM
John, Paul (Ringo?...),
Thanks for your informative posts. Both of you obviously have more fundamental knowledge about electronics than I do. Your input confirms that the behavior of a tube rectifier hasn't been matched by the Copper Cap.

True, I didn't measure total current. I only compared plate voltage drops between the tube rectifier and Weber products. I also had a similar set of numbers with a Carr Slant 6V, which has a pretty stiff power supply. The Deluxe Reverb is hotrodded with 35mf/700v first stage, larger 100 ohm choke, and 180ma current power tranny.

I am a tinkerer and recipe follower who has managed to get good results so far (and not electrocute myself!) using my limited knowledge. I especially value input from guys with the depth of knowledge that you have. By the way, Paul, the "fixed-bias" Princeton Reverb phase inverter mod attributed to you was a major improvement over the 'balanced resistor" mod (that was the previous Princeton Reverb "mod of the week.") Thanks for that!

I always prefer to buy from "Mom 'n Pop" or "Father 'n Sons" kind of businesses when I get a chance. Paul, you are absolutely right about both Webers being nice folks and very reponsive to their customers. I spoke with Ted Sr. for over an hour one night in the early days of their business and was very impressed. I have bought about 10 speakers from them over the years, and had several EV SROs reconed with excellent results (Well, not including the mixup fiasco where I was sent somebody elses speakers, or the wrong impedance voice coil was used...)

Unfortunately, in my experience, their quick responsiveness about a problem sometimes comes with a subjective, almost defensive edge of denial. As in this case. This is with respectful and objective comments given by me, not "your product sucks" sort of vindicativeness.

For example, I have had three new Weber speakers with voice coil rubs new out of the box. "Oh that's UPS's fault. They handled the speaker too roughly." was the standard reply. (Perhaps the voice coil wasn't centered properly or the coil/magnet gap is too small to be practical?) I've never had a voice coil rub with any other new speaker brand, and I've bought and sold at least two dozen besides the Webers in the past few years. Practically every Weber speaker I've owned, including Michigan 12 and 10's, had cone cry.

I'm not trying to hurt Weber's business. I've got nothing personal against them. I've spoken with others who have had nothing but 100% positive Weber purchases, and also others who've had an ongoing experience similar to mine. That I like the Webers and want them to succeed doesn't excuse the fact that their products haven't always performed as advertised. That they sometimes deny the problems forces me to the public forums to see if others have experienced the same thing.

I'm far from the only person who does this. One of the purposes of these forums is to give each of us a chance to benefit from other person's experience. I've been greatly enriched by this.

What I've learned in this instance is that when it comes to Weber Copper Caps, the old saying "looks good on paper" applies.

Peace and Harmony,
Ken

John Phillips
03-31-2003, 10:03 AM
Paul - yes, I did check the PT secondary voltages. I measured the AC then multiplied by sqrt(2) - having first checked that the meter was giving the correct reading like this by temporarily fitting a cap directly to the output of the rec without the rest of the amp connected (I had to physically break the circuit here to measure the current anyway, since the amp has no HT fuse) and measuring the DC on that.

The test voltages I measured were secondary AC x sqrt(2) at the tested current, and first filter cap voltage, so the difference should be purely due to the rectifier.

I did not try at idle and full power - in this particular case it doesn't matter, because this amp is actually true Class A (one of the few...!). It says it's 30W but actually puts out about 25, and will tolerate being biased to give true dual full-waveform amplification (just), although as I said I had to rebias it very slightly to keep within tube dissipation limits; I raised the cathode resistor from 250 to 270 ohms. That's why I picked this amp to test (and because I had it in bits anyway!).

There was however a definite PT sag as the tube current was increased. As the current went from 0 to 137mA the secondary dropped 12V, which is quite a lot, and in a Class AB amp you're definitely going to get some noticeable sag from this.

What I found very interesting is that it appears that the rectifier resistance is not only not constant, but falls with increasing current, and possibly with lower input voltage - although the variation here is much smaller and doesn't seem likely to be the main factor to me. This is actually completely counter to my expectations, and shows that (unlike what I have believed for years) there is some difference between a tube rec and a solid-state one with a series resistance. Hogy in particular will find this amusing since I had an argument with him about it...

I'm still not sure how much practical difference it makes to the sound though. I'd be interested to hear KLB's thoughts on this, since if there isn't a sound difference, why worry if the technical characteristics aren't quite the same? As Weber says, they've sold a lot of Copper Caps and most users appear very happy with them.

KLB
03-31-2003, 10:33 AM
John,
Oh, I am supposed to make MUSIC with these things?
Ha! OK, here are some subjective comments...

In comparing the SOUND of the Mullard GZ34 and Webber WZ34 Copper Cap, the Deluxe Reverb with JJ 6L6GC was biased at 17.5w idle dissipation. The JJ are not my favorite sounding 6L6, but they sure have power and punch. The speaker is an Eminence M12 rated at 120w. Here are the bias and plate voltage points again:

GZ34 = 41ma @ 428v, 201v on V1, pin#1
WZ34 = 46.5ma @ 376v, 176v on V1, pin#1

Sorry, I don't have the tools to measure the RMS output with each rectifier.

First the GZ34. More headroom, spank, harmonics. Tighter bass. LOUD when I got it to distort. Smoother highs. Wow, I've never heard my Deluxe Reverb sound like this.

WZ34. Early breakup, sort of ragged top end, lots of sag, flubby bass (using same settings as GZ34.) A useful "swamp" tone, but I wouldn't want the amp to sound like this all night! The high end was almost ugly. It made the powerful and punchy JJ 6L6GC sound like an old pair of 6V6 biased really hot.

I'm convinced that SS rectification components -- especially under load -- can introduce unpleasant high frequency distortion overtones unless better quality diodes and before/after snubber networks are used. I learned about these things from Bottlehead.com. According to threads I have read and online descriptions, Ted Weber apparently doesn't think any of this makes an audible difference.

Copper Caps get very hot, just like a tube rectifier. I suppose this is due to their tossing away so much of the power supply current and voltage drive as heat. So there goes some of "the amp runs cooler" benefit. I didn't run the amp long enough to know if the power waste through the Copper Caps causes the PT to heat up as if the amp is driven hard.

The heavy sag and early breakup with Copper Caps will no doubt appeal to some.

I compared these "equivalent" rectifier products and discovered they weren't at all the same. I am sending the Copper Caps back for a refund.

Hmmm, I wonder how the new JJ 5AR4 compares to the Mullard?

Peace,
Ken

hunter
03-31-2003, 04:38 PM
As I recall, is Ted Weber did solicit actual rectifier data and compared the performance of the Cap to various manufacturers tubes and tube spec data for a range of currents/voltages. I remember the posted results correlated well with both spec data and actual tube performance. Your comparison results with only one rectifier tube do not mean the entire Copper Cap product line is faulty. I have subbed copper caps in numerous amps equipped with various tube rectifiers of different mfg origin. I have found the cap to produce values both lower and higher and on rare occasion the same as the tubes they replaced.

It is possible that Mullards have lower forward resistance than spec (giving headroom, clarity etc and adding to the sonic goodness “reputation”). I have pulled a couple of old Mullards and found much smaller voltage differences than you saw (<=10) when subbing in Caps. I tend to think that the WZ Copper Cap is probably performing pretty close to spec for a GZ rectifier and I tend to believe the Cu Cap befiore I believe the tube in this case. Of course there is always a chance the copper cap you received is not working properly. I’ve used a bunch of these things and only once have I ever heard any difference and it was a slight improvement. The tube I removed was one of those fake GZ34s with low output. The amp tightened up slightly with the cap. Heat given off by the Cap is no different from heat given off by a tube with the notable exception of the filament heat. The real issue is the filament supply. By eliminating the filament a significant load is taken off the tranny. That is the heat benefit that is gained, not a change in heat given off by the rectifier.

As far as the speaker comments, it is kind of like climbing out of a sports car and complaining because the steering is stiff and quick and the ride is rough. Weber makes a high performance speaker with tight tolerances designed to respond to the amp (which isn't always a great thing admittedly). If you read the info that comes with each new speaker, Weber explains that tolerances are very close and careful handling is required to avoid rubs. If you want a less responsive speaker that will not produce cone cry, they can add doping to prevent or minimize it. I have had a couple of rubs on Weber speakers too. One I bought used and apparently it was damaged in shipping from the owner. The other one developed a rub in the amp while playing. Even though I wasn’t running it hard it got a rub. I attributed it to the tight tolerances. Also care must be taken to tighten the mounting screws to avoid twisting the frame. Based on that experience, compared to other mfg speakers I have found Weber’s more sensitive to rubs. That can be a pain but the tradeoff is improved efficiency and response.

As far as John’s analysis I’ll have to think about it and I probably won’t be smart enough to figure out exactly what he is seeing. However looking at the at the tube data sheet, I find the current/voltage charts are non-linear. The highest amount of non-linearity is at the low (<70 ma range). As an example (for a given set of transformer voltage and resistance conditions) increasing the current output from 175 to 200 ma produces a 11 v drop. Increasing the current from 25 to 50 ma produces a 20v drop. If I'm thinking straight it sounds like a higher forward resistance at low currents just like you saw. And the differences seen between the Mullard and the Cap are much more dramatic than the non-linearity could explain. However, I wouldn’t speculate that Mr. Weber was unaware of this or didn’t take it into consideration in his design.

hunter

KLB
03-31-2003, 05:03 PM
Hunter,
Thanks for your observations and sharing your experience.

In a polite e-mail to Weber over the weekend, I carefully explained the different measurements between the tube GZ34 and copper cap WZ34, and also tried the 55v/175ma version to confirm. I suggested the possibility that these weren't up to spec. I posted Weber's response at the start of the thread. The only option they gave me is to get a refund. The product works. Other people like it. Period. No other recourse given.

I wanted the Copper Caps to work for me. I would prefer there actually was a problem these particular ones so I can get replacements that perform according to spec.

Before ordering the Copper Caps, I was warned by someone else that the ones he tried weren't anywhere near spec. He said that Weber wasn't receptive to that fact. I was ordering the new C1265 (which I like!), so figured I'd try some Copper Caps.

As for the Mullard being a super duper rectifier, sure they are good. Mainly, it's because they are well built, last a long time, and they don't rattle. The various Chinese and Russian 5AR4 I have tried give similar static voltage drops if the amp is setup for the same current draw. Too bad that many of these rattle in combo amps. I plan to try the new JJ 5AR4.

The only GZ34 tube rectifier I recall being wildly out of spec compared to the Mullard was the Sovtek 5AR4 from a few years ago that apparently had 5Y3 innards. A while back, I was able to get a JTM-45 reissue head off the net for $425 because the owner just couldn't get it to sound right. I discovered the plate voltage was 350v! Yup, it had this Sovtek "5Y3 in 5AR4 clothing."
Popped in a real 5AR4, rebiased, and Whammo!

Don't ask me how much money I lost when I sold the Roland VG-88....

Ken

hunter
03-31-2003, 06:19 PM
As John said, if you are really interested in getting a Cu Cap to work for you, you can spec the voltage drop you want (just be sure you really know what your total current draw is). In your case you could use your Mullard data as your benchmark. I don't even think there is added cost to do this. I think a variable version is available or in the works that would allow you to sdjust your plate voltage to suit your taste. That is if you really want to use one. Of course if you can hear the high frequency distortion generated by the solid state components in your power supply none of this matters i guess. Fortunately for me i can't hear it.

Even eating shipping you can experiment and buy a boatload of Cu Caps for the cost of a single Mullard and the Caps never wear out. FWIW the only time I have had an amp of mine go down on me at a gig in a lot of years, knock on wood here, was a rectifier failure (and yes it was a Mullard). I always carried a little solid state plug in for emergencies but no more (well it is still stuck in my gig bag but I won't be needing it). My experience is the Cu Caps perform as advertised.

hunter

John Phillips
04-01-2003, 03:44 AM
Ahh... why didn't I spot this earlier? KLB, it's no wonder you got different sounds from the GZ34 and the WZ34 - you're running the 6L6s under wildly different conditions.

You set them by power dissipation, which is not at all an accurate way of biasing a tube (it's only really useful as a safety check). By setting the idle current at 46mA with lower plate voltage, this is FAR 'hotter' in terms of operating condition (even though the dissipation is the same) than 41mA with higher plate voltage. This alone could explain the swampy, mushy sound. I should have realised this sooner since I dislike running tubes hot for this exact reason - I prefer the cleaner, bolder sound when they are as cold as reasonable without introducing crossover distortion. This is actually how I set bias - bias cold until distortion occurs, then back off until the amp sounds good. I only check the current (and calculate the power) to make sure it's in the safe region - I never 'set' by current.

Setting by current can also be misleading - again, it isn't accurate because you need to know the other operating conditions. To make a true comparison between the recs, you need to set the power tubes by operating point, ie where the waveform clips relative to the input signal. With slightly lower plate voltage, this might even occur at lower tube current, not higher. This in turn will actually raise the plate voltage a little.

If you do that, you may find the Copper Cap is closer sounding to the GZ34 than you think. It's obviously not going to be precisely the same, but as has already been said there may be some variation in GZ34s.

I'm not totally sure about the switching transients in solid-state diodes. I've heard alot about this but I'm not totally convinced - the filtering should take care of it. It may be necessary to fit a smaller-value non-electrolytic cap in parallel with the main filter cap though, since electrolytics have relatively high internal resistance and so can't suppress very high frequencies well. You see this in hi-fi amps quite often.

KLB
04-01-2003, 08:09 AM
Originally posted by hunter
(snip…) you can spec the voltage drop you want (just be sure you really know what your total current draw is). In your case you could use your Mullard data as your benchmark. (snip) That is if you really want to use one. Of course if you can hear the high frequency distortion generated by the solid state components in your power supply none of this matters I guess. (snip)

(snip) the only time I have had an amp of mine go down on me at a gig in a lot of years, knock on wood here, was a rectifier failure (and yes it was a Mullard). I always carried a little solid state plug in for emergencies but no more (well it is still stuck in my gig bag but I won't be needing it). My experience is the Cu Caps perform as advertised.

hunter

Hunter, thanks for your comments.

In the case of the WZ34 Copper Cap, I did spec the voltage drop I wanted: To be a drop-in replacement for a tube GZ34 and to be interchangeable with similar results.

The GZ34 and WZ34 specs are 17v drop at 225ma. In my test, at 41ma per power tube, plus the minor current draw from the cathodes of the 4-12AX7 and 2-12AT7, the total current draw at idle was far less than 225ma. As you see from earlier posts, when I sub a WZ34 for the GZ34, the bias point and plate voltages shift significantly.

Standby/Plate/Bias current

GZ34 - 477v/428v/41ma
WZ34 - 475v/401v/29ma

The current draw isn’t approaching 225ma in either case, nor is it taxing the power transformer, which has a 180ma current capability. When I re-biased to try get the equivalent current draw with the WZ34, the plate voltage drop was even greater.

So, as you see, my experience and testing shows the WZ34 is not a drop-in replacement for a GZ34.


Originally posted by unquiet
Ahh... why didn't I spot this earlier? KLB, it's no wonder you got different sounds from the GZ34 and the WZ34 - you're running the 6L6s under wildly different conditions.

You set them by power dissipation, which is not at all an accurate way of biasing a tube (it's only really useful as a safety check). By setting the idle current at 46mA with lower plate voltage, this is FAR 'hotter' in terms of operating condition (even though the dissipation is the same) than 41mA with higher plate voltage. This alone could explain the swampy, mushy sound. I should have realized this sooner since I dislike running tubes hot for this exact reason - I prefer the cleaner, bolder sound when they are as cold as reasonable without introducing crossover distortion. This is actually how I set bias - bias cold until distortion occurs, then back off until the amp sounds good. I only check the current (and calculate the power) to make sure it's in the safe region - I never 'set' by current.

Setting by current can also be misleading - again, it isn't accurate because you need to know the other operating conditions. To make a true comparison between the recs, you need to set the power tubes by operating point, i.e. where the waveform clips relative to the input signal. With slightly lower plate voltage, this might even occur at lower tube current, not higher. This in turn will actually raise the plate voltage a little.

If you do that, you may find the Copper Cap is closer sounding to the GZ34 than you think. It's obviously not going to be precisely the same, but as has already been said there may be some variation in GZ34s.

I'm not totally sure about the switching transients in solid-state diodes. I've heard a lot about this but I'm not totally convinced - the filtering should take care of it. It may be necessary to fit a smaller-value non-electrolytic cap in parallel with the main filter cap though, since electrolytics have relatively high internal resistance and so can't suppress very high frequencies well. You see this in hi-fi amps quite often.

John,
As I explained above, it was not my intention to run the amp any differently with the WZ34.

There has been much debate about how to bias tubes. In the end, bias is set to achieve a desired sound and feel. As you said above, you also use measurements to get you in the range, then your ears are the final judge. As long as the plates aren’t glowing red, there is no short-term reliability issue, though expected tube life and long-term reliability increase the colder the tube is biased.

The amp sounds a great deal different when running at 41ma and 428v on the plates (GZ34) versus 29ma and 401v (WZ34). I simply could not duplicate the sound with the GZ34.

With the 55v/175ma Copper Cap I was hoping to intentionally lower plate voltage and increase bias current so I could operate the amp closer to Class A. The actual voltage sag under less than maximum current conditions was greater than expected with both Copper Caps.

Regarding SS vs. Tube rectification, the audible differences are most apparent under load (amp distortion) conditions, especially when the incoming AC is dirtiest and/or RF interference is strongest. Theoretically, the diode switching occurs at such a high frequency it shouldn’t matter, but in my opinion, audible artifacts are created in the range of hearing. It makes transients and distortion overtones less pleasing. This is more a side issue here because I couldn’t get the Copper Cap to Tango in the first place!

Cheers,
Ken

teleharmonium
04-14-2003, 11:59 AM
good discussion, thanks to all.

unquiet wrote: "I did not try at idle and full power - in this particular case it doesn't matter, because this amp is actually true Class A (one of the few...!). It says it's 30W but actually puts out about 25, and will tolerate being biased to give true dual full-waveform amplification (just), although as I said I had to rebias it very slightly to keep within tube dissipation limits; I raised the cathode resistor from 250 to 270 ohms."

I'm blown away by the revelation that the Zodiac is true class A. Is this the case with the stock wiring, or only after you changed out the cathode bias resistor ? It's a shared resistor for both power tubes, right ?
I knew that it put out low power for a pair of EL34s but I thought that was only due to the tubes being cathode biased. A Zodiac 30 is my #1. I have lately been running it with a GZ37 instead of a GZ34 to drop the plate voltage a little, so I can more safely run Siemens (RFT) EL34s, which I like for their tone. I know it works the power tranny a little harder, but it's a big Partridge, I figure it can more than take it.

John Phillips
04-14-2003, 12:46 PM
I think it is a true Class A. Whether it was designed like that on purpose I don't know. If so they must have deliberately ignored the power rating for EL34s.

Certainly with the original 250-ohm cathode resistor (which is common to both tubes), the tube current was independent of signal level right up to clipping, showing that the full waveform is being amplified completely by each half of the push-pull - which is the definition of Class A (I don't have a scope at the moment to really check... it died a while ago and I haven't got around to finding another... I really should). But at this it was stressing the tubes too much really - 31W is well over the max, and both tubes were showing some plate glow. Rebiased with the 270-ohm resistor, it was still very close - probably still just on the edge of Class A.

I do think these are great and very under-rated amps. For some reason they had a bad reputation by the late 60s - I really don't know why, as they are very well-made, reliable and IMO excellent sounding. The "Croc-skin" ones are also so cool-looking, but maybe not by the tastes of the later 60s... likewise probably tastes in tone had changed a lot.

The ones that followed, that looked and sounded more like Fenders - the Treble & Bass 50 in particular, which is a very close copy of the BF Bassman but with EL34s, is an incredible-sounding amp - were already starting to show evidence of the styling crimes that ended in the T&B series with the full-height aluminum panels by the early 70s. These are still solid and good-sounding, but just look so awful that no-one wants to know. Over here you can still pick one up in fully working order for £50 or less, usually including a full set of Mullards...