Plate Voltage eluding me !

[hamfist]

Hoping for a small bit of advice on measuring the plate voltage on a new amp of mine. Have asked elsewhere but without success as yet.
It's a 1974 Wilsic (Miles Platting) V50. 2 x EL34's, 3 x 12ax7's, 1 x GZ34 very clean, loud 50W.
I picked it up for a very good price, and all is well with the amp. It sounds great too.
However, just for thorougness, I wanted to check the bias. have biased a number of Marshalls and my Orange Rockerverb before, so I thought it was going to be easy.
The main problem is that I can't seem to measure the plate voltage. If I try measuring the voltage between pin 3 and the chassis, all I get is a huge, continuous static noise through the speakers the moment I touch the pin 3 socket with the probe of the DMM. Both channel volumes are right down BTW.
Same happens if I measure between pins 3 and 8.
I'm no expert on these matters, however I am used to biasing and changing components in hand-wired amps, so I like to think I'm fairly safe.
If it helps, here's a gut shot and schematic of a V50. Please note, the gutshot is not of my amp, but an identical one.

http://w1.278.telia.com/~u27805779/images/MilesPlattingV100/Schematics/MilesPlatting-50.jpg (http://w1.278.telia.com/~u27805779/images/MilesPlattingV100/Schematics/MilesPlatting-50.jpg)

http://w1.278.telia.com/~u27805779/images/MilesPlattingV100/V50/miles%20009.jpg (http://w1.278.telia.com/~u27805779/images/MilesPlattingV100/V50/miles%20009.jpg)

Anyone any ideas what is going on ? As I said before, the amp is working and sounding just fine.

Oh, also, can anyone advise me on fuses (for UK 240V). The amp has two external fuses, as you can see in the gutshot. Currently, these are both 5A 250V fast blow fuses. The schematic indicates that the fuse between the elctrolytic and the rectifier should be 500mA, but I don't know whether a fast or slow blow fuse is needed.
The other (mains ?) fuse is surely rated too high at 5A ? My other amps have generally had 2 or 3 amp fuses. Again would a fast or slow blow fuse be needed, and what rating would anyone suggest ?

The amp is so rare that there seem to be no other info on the www about it.

[Groovey Records]

Ham,

This is not my forte but wouldn't Pv be measurable on the same pin no matter which amp it is installed into? ie 3 & 8

Do you have another amp to check the tubes in or measure Pv?

Still the amp may be cathode biased which would effect your readings.

I found two reviews for your amp on harmony central maybe these guys could help (long shot) if their email addresses are active

http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Miles+Platting/V.50/10/1

i

[Blue Strat]

It's not unusual to hear SOMETHING when you probe pin 3. Volume controls would have no effect since they're way back in the early part of the preamp.

The best thing to do is CLIP onto pin 3 so the probe isn't moving around. Once you've done this the noise should stop.

Generic warning: The voltage on pin 3 can kill you.

BTW, I'm assuming that you realize that pin 3 has nothing to do with the actual bias current but is only used in the calculation to find the bias current range. If you assume that it's 450V, you probably won't be off by more than 5% and 5% is more than close enough for your calculations.

[hamfist]

Thanks guys for the responses.
Groovey - the amp is not cathode-biased, so I can rule that one out.
I have measured PV in my Rockerverb, and also other Marshalls I have owned - never a problem. I don't have another EL34 amp at the moment, so can't try the tubes in a different amp. But the amp sounds great, so I don't suspect the tubes.
I may need to contact someone else with the amp, there is a guy at PP with one (or even two actually !).

Blue Strat - Yeah, I'm used to a "click" when the probe touches pin 3, but this noise is BAD ! I did "go for it" and held the probe firmly onto the pin, but that firm contact did not help at all - I thought I was going to blow my speakers !! I'll try clipping on when I have the amp open again.
As I couldn't find the B+, I actually went on the safe side and assumed a B+ of 490V, until I know different.

Also, any idea aout the fuses question in my original post ?

[scottl]

Measure B+ at the filter cap......

[hamfist]

Measure B+ at the filter cap......

OK, no probs. Where exactly do I measure from ? I can do it safely, just need to know which cap, and which contact, has the B+.

[JohnLochner]

Not sure if this helps, but I biased a Marshall 1959SLP (reissue) one time that was really picky about where the meter leads were positioned when measuring plate voltage. It would go into high-freq oscillation if the leads were just left hanging when measuring. I used some really short leads that I had made up and it fixed that problem.

[BJF]

Hi,

You might want to play this safe and use a Biasprobe. It is likely that you cause an oscillation with your DVM attached at pin 3. This can harm the output transformer.

The B+ is likely lower than 490V's, perhaps in the vincinty of 400V's and the amp would be fine running something like 25-35mA.

As you can see in the schematic the first filtercap is rated at 450V and so voltage across that cap must be lower.
You can see in the schematic that HT fuse is rated at 500mA and a slow blow will do just fine.

For the mains fuse use something like 1,25 mA and slow blow will do just fine.

Since this is a Miles Platting you will have to have the chassis out from cabinett to attach bias probe.

To measure current through the powertubes you can measure the voltage across the 200 Ohm and withdraw the current through the preamp which you can measure or estimate. This is how bias would have been set at the factory.

If you are the least suspicous of making this measurement then get a probe

Have fun and be safe
BJ

Affiliations
BJF Electronics
Sweden
www.bjfelectronics.com
www.mpamp.com

[hamfist]

Hi,

You might want to play this safe and use a Biasprobe. It is likely that you cause an oscillation with your DVM attached at pin 3. This can harm the output transformer.

The B+ is likely lower than 490V's, perhaps in the vincinty of 400V's and the amp would be fine running something like 25-35mA.

As you can see in the schematic the first filtercap is rated at 450V and so voltage across that cap must be lower.
You can see in the schematic that HT fuse is rated at 500mA and a slow blow will do just fine.

For the mains fuse use something like 1,25 mA and slow blow will do just fine.

Since this is a Miles Platting you will have to have the chassis out from cabinett to attach bias probe.

To measure current through the powertubes you can measure the voltage across the 200 Ohm and withdraw the current through the preamp which you can measure or estimate. This is how bias would have been set at the factory.

If you are the least suspicous of making this measurement then get a probe

Have fun and be safe
BJ

Affiliations
BJF Electronics
Sweden
www.bjfelectronics.com (http://www.bjfelectronics.com)
www.mpamp.com (http://www.mpamp.com)

Excellent information BJF. Thanks very much !!
I see now that the B+ must be under 450V due to the caps ratings. I’ll just play it safe and bias fairly conservatively I think. The amp sounds great with plate current of 28mA anyway, which is well within spec for an EL34, so I’ll just leave it at that I think.
I have actually been using a bias probe, but my probe only lets me measure plate current not plate voltage – it’s one of those cheaper SRS ones – still does the job well, but just means you have to get the B+ some other way.
Your info about fuses is superb. I shall get some ordered today.

[phsyconoodler]

It sure sounds to me like you don't have your meter on the right voltage RANGE.Thaht amp may have a DC plate voltage of over 630v and your meter may not have the range to test it.Confirm this before continuing.
Even if it is high voltage,your bias setting is not bad.
Because the caps may be rated at 450-500v,you may have a pair of caps in the first position that add up to 900-1000v.

[hamfist]

It sure sounds to me like you don't have your meter on the right voltage RANGE.Thaht amp may have a DC plate voltage of over 630v and your meter may not have the range to test it.Confirm this before continuing.
Even if it is high voltage,your bias setting is not bad.
Because the caps may be rated at 450-500v,you may have a pair of caps in the first position that add up to 900-1000v.

Fair point. My meter goes up to 600V. Could someone who understnds these things have a look at the schematic, linked to in the initial post, and confirm whether it is possible that there may be a pair of caps paired up to give 900V. this is beyond me. :o

[phsyconoodler]

I looked at your schematic and it looks like about 450v on the plates and no caps in series,so that's not an issue.

[BJF]

Hi,

Well, you can put two (or more) capacitors in series to get a higher voltage composite capacitor and if so equalizing resistors should be employed to ensure equal charge.

However, in this amplifier as per the supplied schematic the output of the rectifier tube feeds just one capacitor of 32µF/450V- there's a limit to how large a capacitor the GZ34 can charge- but then this is then followed by a 200? resistor that is decoupled by another 32µF capacitor and thereby increasing powersupply rejection rate eqvivalent to a silicon rectifier driving a lot larger capacitor. It is an unusal connection in guitaramps and mostly otherwise found in Gibson amps

Simply as the output of the rectifier feeds a capacitor with a rating of 450V's, no designer would allow more than 420V's across such a capacitor and that would be stretching it since the capacitor would then get a short life so a voltage in the range of about 400V's would be more like it. That's an educated guess.

Oh and you can measure the voltage at the output of the rectifier just incase you are curious-that's over that 32µF/450V capacitor.

There's not much information available on the Miles Platting amps, but I can tell you I once had the 100W version in for service and the owner asked what I could tell, and as far as I know Slade endorsed these amps.
Distribution of these amps was unfortunately not very effective and the company vanished.

I'd say these amps show interesting combinations of circuits popular: kind of like a mix between a Fender and a Marshall just to give a hint.

I would not worry much if the amp works and I think I know how your amp sounds ;) having once been fortunate to play a Miles Platting.

The two 100nF capacitors that are the outputs of the phaseinverter may in time need replacement.

Have fun and enjoy
BJ
Affiliations
BJF Electronics
Sweden
www.bjfelectronics.com
www.mpamp.com

[hamfist]

Thanks physconoodler.

And again, BJF, much appreciate your input !! I have to say, this amp has a great clean tone, and the EQ is very effective.

[audiozone]

i took a quick look at the schematic. i see two dc supplys from the transformer to the power tubes. it doesn't give pin numbers, but i'm assuming pins 3 and 4. the plate is obvious, so the other is screen grids, and the suppressor grids go to ground. is it suppose to be ultraliner?{probably not}, or are the screen grids suppose to be the same voltage as the plates?{unusual?} i'd guess thats why you are gertting so much noise when testing.

also your power transformer is labled 350-0-350 and feeding a gz34 tube 350v x 1.3 = 450v

[hamfist]

i took a quick look at the schematic. i see two dc supplys from the transformer to the power tubes. it doesn't give pin numbers, but i'm assuming pins 3 and 4. the plate is obvious, so the other is screen grids, and the suppressor grids go to ground. is it suppose to be ultraliner?{probably not}, or are the screen grids suppose to be the same voltage as the plates?{unusual?} i'd guess thats why you are gertting so much noise when testing.

also your power transformer is labled 350-0-350 and feeding a gz34 tube 350v x 1.3 = 450v

You've lost me a bit audiozone but, yes, I have heard the word "ultralinear" in the context of discussion about this amp. Is this the reason that I am getting noise issues trying to measure the B+ ?

[BJF]

Hi,

Yes that is indeed funny- in very small text inside the powertransformer drawing it says 350AC!
If voltage across this first 32uF /450V cap were indeed 450V and at the ambient temperatur inside the rather small cabinett of a Miles Platting, this cap would have a life expectancy of roughly two to six months........

Yes, schematic shows what would only be interpreted as ultralinear circuit, and in the chassis picture this is also clear following the wires to the output transformer.

Ultralinear is just a way of connecting powertubes to an output transformer and the details on how that works and possible benefits are readly available. It is unusual to find this circuit in guitar amps.

Oscillation during measurment is likely due to the lenght of the test cables to the probes and the position the ground is taken from. This has likely nothing to do with that poweramp operates in ultralinearmode but more to due with the general layout of the amplifier as a whole.

In the picture voltage reading could likely be taken at the fuseholder, sitting to the left of Voltage selector,black wire.

If voltage across this mentioned cap is 420V or higher, I'd recommend you'd change the cap for a 500V or a 550V type and at least 85 degrees Celsius, preferably 125.

Yes, sound as I recall is strong and bold and these amps take pedals well.

Have fun
BJ
Affiliations
BJF Electronics
Sweden
www.bjfe.org
www.mpamp.com