Advice needed: first experiences.

[milkypostman]

For reference, the schematic is here: http://www.freeinfosociety.com/electronics/schematics/audio/redbearkmk120.pdf

So, last night I decided to mod my Red Bear MK60 (JCM800 circuit) with a few things i had read online. On the low input I replaced the 470k resistor and the 470pf capacitor with two 68k resistors in parallel. I also replaced the 470pf capacitor in the treble line of the tone stack with a 270pf capacitor. While doing this I was curious if there was any magic going on under the board. The board you can see here but I don't know that this helps anything. But just in case you're curious (please ignore the circled parts):
http://i28.photobucket.com/albums/c207/decurtis/possiblezener.jpg

Now, my first mistake is that after doing this I didn't rebias the amp. I honestly didn't think this would matter. Maybe it doesn't but here is the weird thing. I plugged in and started playing. Loved the changes and it sounded great. After a while it started to break up even on lower volume tones and I could smell something burning. PANIC! The amp was sitting on a 2x12 and so it was a little lower and man, the tubes were bright orange/red.

So, I figured that this threw something off and I needed to rebias. I went through the work of adding a 10ohm resistor (all I had at the time) between pin 8 (which is incidentally wired to pin 1) and ground. The reading just kept going up and up and up. Since this is 10ohm I'm expecting readings around 300mv (because I'm using 10ohms instead of 1ohm) and I was getting upwards of 1V! at which Point i shut the amp down.

I thought I really f'd something up and the only thing I could thing was to put the tone stack to original, so I put back the old 470pf capacitors (which are actually a 800pf and 900pf capacitor in parallel as you can see they are the orange boxed caps in the picture) and then tried biasing again. I could NOT get this thing under control. Still the bias readings across my 10ohm resistor were going up and up and up and I had to shut off the amp again. I even checked the bias voltage and it was like 1V positive, not even near -50V. However, the plate voltage was perfect (spot on 495V).

So, I bought this amp for 200bucks and I have decided it's my experimental amp but still sucks to feel like you've wrecked something bigtime (big $$$) so to say the least I was a little anxiety filled.

This morning I woke up and started from the start. Plugged in the amp, before turning off standby I checked the negative bias voltage. -50V. Great. Then I flipped the standby off and check the current through the tubes. I was able to use the variable resistor on the bias circuit to get anywhere from 170mv to 310mv (which translates to 17ma to 30ma -- remember at this time i have a 10ohm resistor) which I think is about right. The amp has two Sovtek 5881WXTs which I think are 25watts and from all I can gather, are meant to be run around 25W max, and 495V*.03A ~17W). Regardless, I couldn't get much higher. I think the previous tech did some work here to prevent the tubes from getting pushed too far by lowering the resistance before the adjustable resistor. There has been significant work done on the -V (bias circuit?) circuit by the previous tech who put in a Super Reverb Power Transformer so there are a few added resistors in parallel with the old resistors. While I was biasing I did re-check the plate voltage on pin 3 and the bias voltage and all seemed kosher. However, I'm only biasing on one tube, I am planning on putting a 1ohm on the other tube tonight and checking that they are relatively close.

As a side note, before I get any word about taking it to a tech, 1) I am learning, and 2) I am trying to be as safe as possible. I wait until there are only a few mA of voltage across each capacitor before I begin work. Except for when I'm biasing where I'm using rubber covered alligator clips and normally don't move them during my tests, so except for my screwdriver (small one covered in electrical tape) on the bias control, I keep my hands out of the amp. I do not want to die.

Anyways, the question is what could've caused the tubes to red plate and why it seems OK now. Is it possible in messing with the eyelet board I loosened something up that jacked up the circuit? But doesn't make sense that it would then be perfect this morning. I even played it for a while. Does a capacitor change on the tone stack really effect the bias that much??? Also, does my approach for biasing look right? I mean, 30mA across a 10Ohm resistor would be 300mV? I'd like to change the treble capacitor back to a 270pf...


Thanks in advance.
I appreciate any feedback that could help educate me a bit more.

[phsyconoodler]

Changing a tonestack resistor has no affect on bias whatsoever.But...you clunking around inside the amp is obviously what caused your bias problem.Check all your grounds.
I find removing the treble peaking circuit makes the amp sound too woofy.

[milkypostman]

What do you mean removing the treble peaking circuit? You mean lowering that capacitor value?

I am also going to raise the resistance that goes to the bass and mid pots so as to lower the low-end. These amps are notorious for a being too woofy.

[phsyconoodler]

The 470k/470pf resistor/cap that you replaced is the treble peaking circuit.You need that to make the amp less woofy.
A true JCM800 is anything but woofy,so there is something else making it that way.What are the values of the bypass caps and the coupling caps in your amp?

[TweeDLX]

Basically, what you're doing is "Fenderizing" it. The 470pf/470K resistor combo on the input is your "treble peaking" circuit. Like psycho says, the changes you made have absolutely no effect on bias. You may have a tube issue (unlikely), or a component in the bias supply going south. Check your tube sockets for stray "whiskers" of wire that may be making contact with other pins. Retrace your work on the board to make sure you didn't accidentally unsolder something, or push something far enough to make contact with another component.

[milkypostman]

Time to let my inexperience shine: I'm not sure I know which are the bypass caps and which are the coupling caps. I'm sure I could find them just don't know the terminology. Are the bypass caps what I would call the filter caps, i.e. filter the incoming DC current from the power supply? I think they are 100uf and 50uf @ 150V

Yes, TweeDLX you're correct. I am doing some Fenderizing. Is this forbidden? (joke)

I like what I've gotten so far. The treble peaking circuit change made a huge difference in a good way for me. I would say it's warmer. Less shimmery than before and seems to give me more treble control in my tone stack.

It's possible this thing needs a re-capping but I'm really not wanting to do it. The installed caps are old-school russians where the outside shell is the negative and the middle is the positive and they screw on. My only option is do put marshall style mounted ones in and drill a bunch of holes, or grab a turret board, attach it to the chassis, and run wires through.

[TweeDLX]

Bypass caps are usually attached to the cathodes of the pre-amp tubes (pins 3 & 8 on a 12AX7 type). Coupling caps connect the gain stages to each other, usually attached to the plate load resistors at one end. The caps you mentioned in your post are the filter caps (part of the power supply). I sent you an email with a pic, and the link below has some good info for beginners. http://www.diystompboxes.com/pedals/tubedummy.html

[milkypostman]

The 470k/470pf resistor/cap that you replaced is the treble peaking circuit.You need that to make the amp less woofy.
A true JCM800 is anything but woofy,so there is something else making it that way.What are the values of the bypass caps and the coupling caps in your amp?

Hey phsyconoodler, have a look at that schematic. Except for the power section, which was modded by a professional tech to take a more modern power transformer, the amp is stock so the bypass caps and coupling caps should be as per the schematic.

The link with TweeDLX provided should give me a place to start if I want to start playing with the bypass caps or coupling caps. The sensitivity switch on this amp is pretty much worthless, but it's possible I could do some work on the cathode circuit and make it worthwhile. More likely I'm going to put a boost in front of the amp because that's all I was using the switch for.

I am also going to start playing with replacing either V1 preamp tube to a 12ay7 and the inverter tube to a 12at7. Either/OR. I have read this should warm up the sound.

Thanks guys.

[TweeDLX]

Just replacing a 12AX7 PI tube with a 12AT7 may not sound so good. Look at some schematics of amps with 12AT7's for a PI tube and notice the differences between your PI & those.
Bandmaster AB763 with 12AT7 PI (http://web.archive.org/web/20061020104610/www.schematicheaven.com/fenderamps/bandmaster_ab763_schem.pdf)
Bandmaster 6G7-A with 12AX7 PI (http://web.archive.org/web/20061020100900/www.schematicheaven.com/fenderamps/bandmaster_6g7a_schem.pdf)

[phsyconoodler]

That schematic is terrible.I can barely read it.
If you don't know the difference between filter caps and bypass caps then you have no business inside that amp.
You are making changes that make the amp MORE woofy,not less woofy on the bottom end.
If it really was a true JCM800 circuit it would not have the issues you are describing.
The 68k resistor adds to the low end woofiness,that's why Marshall used the treble peaking circuit in the first place.
What speakers do you use and what kind of music do you play?What guitars?
Fenders are described as having farty,woofy bass and you are headed more in that direction.

[milkypostman]

Hrm. Good to know. I wasn't sure how that would change the sound but thought I wouldn't "hurt" to experiment. However, the amp was modded to accept 12ax7s, and originally had rarer 6N2P tubes which has less gain than the 12ax7s. Some people say 12AT7s can be a more accurate representation of those tubes. *shrug*

I do know that the low channel on my amp is *very* close to a 5f6a bassman circuit, and they use a 12ay7 so I figured that might sound good.

I do know the 5f6-a Bassman does have a 12ay7 as the V1 preamp tube which is much lower gain than a 12ax7.

[milkypostman]

That schematic is terrible.I can barely read it.
If you don't know the difference between filter caps and bypass caps then you have no business inside that amp.
You are making changes that make the amp MORE woofy,not less woofy on the bottom end.
If it really was a true JCM800 circuit it would not have the issues you are describing.
The 68k resistor adds to the low end woofiness,that's why Marshall used the treble peaking circuit in the first place.
What speakers do you use and what kind of music do you play?What guitars?
Fenders are described as having farty,woofy bass and you are headed more in that direction.

haha, I figured at some point someone would tell me I have no business inside the amp.

[TweeDLX]

What speakers do you use and what kind of music do you play?What guitars?


What pedals? Experimenting is fine, but it's a good idea to know what you want to achieve tone-wise (and how to get there) before you go mucking it up.

[milkypostman]

I'm using a Peavy Predator which is on its last leg. Waiting (and saving) to buy a Jazzmaster.

Running it through a 1936 Lead cabinet with what I'm told is 2 celestian ct100 speakers. It's technically not my cabinet yet, borrowing from a friend and/or trying to craigslist it for him while he's out on tour. I may buy it just to have a cab though. I'd like to put Jensen c12k's in it.

I have been using a Big Muff Pi from Russia, I also used a Tubescreamer which I liked but was someone elses. I am looking at building (all clones) a Rat with Germanium clipping (instead of diode), a Supr Dupr boost pedal, a Fuzz Factory, and a Box of Rock (Yes a lot of zvex style pedals). I am also really liking my friend's Fulltone Catalyst but that will have to wait until I get a big more money and between the Rat, the Supr Dupr and/or the Box of Rock I think I can get a majority of those sounds.

I basically play what would be described as indie rock although it's more garage sounding. Pavement, Built to Spill, Yo La Tengo have been the comparisons music-wise the two times we've played live (long story).

I basically love Fender tones. I don't like breakup to be too compressed. I think people like these redbear amps because they use the high channel, crank up the preamp, and this this really metal breakup. I prefer the low channel with a touch of breakup. I just noticed two things: the highs were just too shrill (thinking crystal like) and the low end was just too much (no control on the knob, it needed to be at 0).

So anyways, I figured the treble peaking circuit change would cut some of the shrill stuff and raising the slope resistor would give the low-end a bit more control. I'm open to suggestions. I was definitely just trying some more Fender-like things I noticed when looking at fender schematics. I had considered just rebuilding the amp with a Bassman circuit (using turret board) but to me that was more of a big-time and big-dollar project that I wanted to undertake. I am attacking this like programming, you don't make a program right off the bat, you first start changing another program (for better or for worse) to gain your own understanding of how things work. Turn the proverbial knobs. (i don't know that the last sentence made sense I just liked it).

[HipKitty]

I'm most curious about whether or not those electrolytic caps were replaced? I've seen too many of those Russian caps that failed, specially the "spark plug" types.....

Also, invest some money and get those pots replaced along with the original output tranny...maybe add a choke and you'll have a great sounding amp

[phsyconoodler]

If you look at a real JCM800 layout,you'll see that it has .02uf coupling caps and Fender uses .1uf. That alone makes a large difference.
Trouble is,most Fender amps experience the very thing you are trying to control:flubby bass.Now the Marshall doesn't have this issue but guys who like Fender tones don't like the shrill highs.
The bypass caps on the cathodes come into play here.Marshall uses .68uf and Fender uses 25uf.A compromise here can help shape your frequency.
For your amp to have both shrill highs and boomy bass may not be the amp at all.It may be the speakers you are using.
Or maybe you are wanting too much from a rather limited circuit.
go to www.ceriatone.com and look at the JCM800 layout and compare your amp to that.They do not have boomy bass and usually the only thing you have to do to tame the highs is remove the bright cap on the volume control(already gone on the ceriatone layout).

[milkypostman]

I'm most curious about whether or not those electrolytic caps were replaced? I've seen too many of those Russian caps that failed, specially the "spark plug" types.....

Also, invest some money and get those pots replaced along with the original output tranny...maybe add a choke and you'll have a great sounding amp

Thanks for the advice. I am a little worried about the caps but still sounds OK for the most part, but it was serviced a while ago when the PT was replaced.

Man, I feel like if I'm going to go that far I might as well just rebuild the whole amp. I think I will eventually when I have more money but I'd end up replacing it with a bassman circuit.

[milkypostman]

If you look at a real JCM800 layout,you'll see that it has .02uf coupling caps and Fender uses .1uf. That alone makes a large difference.
Trouble is,most Fender amps experience the very thing you are trying to control:flubby bass.Now the Marshall doesn't have this issue but guys who like Fender tones don't like the shrill highs.
The bypass caps on the cathodes come into play here.Marshall uses .68uf and Fender uses 25uf.A compromise here can help shape your frequency.
For your amp to have both shrill highs and boomy bass may not be the amp at all.It may be the speakers you are using.
Or maybe you are wanting too much from a rather limited circuit.
go to www.ceriatone.com and look at the JCM800 layout and compare your amp to that.They do not have boomy bass and usually the only thing you have to do to tame the highs is remove the bright cap on the volume control(already gone on the ceriatone layout).

Thanks for the help. The coupling caps on my amp are definitely .022uf. Many people have commented on positive results by changing the slope resistor. One thing that's interesting on this tone stack though is that coming off the mid capacitor is a cap to ground of .0047 which I don't see on the JCM800 or other marshall tone stacks I've seen.

yeah, could definitely be the speakers. I tried another cab a while back (open back as compared to closed back) and it was about the same. The cab I'm using now sounds much better though. The bypass caps on my amp are 15uf so I imagine they're trying to find a happy medium there. I have noticed that changing out the treble peak circuit has helped with the shrill bit.

[milkypostman]

OH MAN! Thanks for that site ceriatone site. I may just gut this thing and put in a bassman circuit. I'll have to do some research to see if I can use the same PT and OT but if I can, 100 bucks would be worth it. I could change out the pots while I'm at it too so that's be pretty sweet. Big project though, not sure I'm ready for that yet.