hacking kit JTM45 bias circuit

[mikepukmel]

Telling us that the caps are 32uF isn't enough. We need to know if they're totem-poled or not. If they are, it cuts their value in half. Unfortunately I don't see them on your schematic clip.

Tiny bias caps went away in the 60s when caps became cheap. FWIW a great many Fender amps commonly used bias supply caps that were larger than the main PSU caps; AB763 BF Twin Reverb (50uF vs 35uF); Super Twins: (80 uF vs 50uF); AA371 Bassman (50 vs 35). My Super Twin got "upgraded" to have 100uF on the bias supply with 50uF on the first filter pole. It hasn't gone up in flames ... yet.

My experience with modeling bias and PSU supply activity shows that the caps come up to voltage pretty darned fast -- typically within an insignificant number of cycles. I think you'd have to have some crazy long time constants before you'd see tubes incinerating at power on. Has anyone actually seen this happen?

edit: Those HiWatt circuits in Mozz's diagram used some pretty large resistor values (82k, 110k) with 100uF bias caps.
What size caps are on the PSU nodes?

Sorry,. the crudely doctored clip of the schematic was made in MS Paint, with a screen shot, and I couldn't get the whole thing at enough resolution in one screen shot. The Valvestorm kit, has lots of tweaks from the 'original' schematic, and they have a copy of the reissue schematic. Anyway, its a GZ34, the first cap is 1/2 of a 32/32, so there's only one 32uf cap between the rectifier and choke.

[mikepukmel]

Ok... On Bob's suggestion I simulated some stuff.

Using a circuit similar to what's been proposed above (120k series, two cap filters divided by a 15k, 47k load) I get:

Pair of 8uf get within two volts of final bias in two seconds and has 30mV of ripple

Pair of 22uf get to within two volts of final bias in 5.5 seconds and has 4mV of ripple

Pair of 100uf take 24 seconds to get within two volts of final bias (and still climbing slowly beyond a full minute!) and, of course, has no ripple worth reporting.

I'd go with 22uf

24 seconds? Seconds? Eek. Thanks. Wow.

[mikepukmel]

I would have expected the standards to improve somewhat over the 50 year span where we've seen the prices of caps reduced so dramatically. But I guess the standards haven't changed as much as I had thought whey would.

looking at the JTM45 we see that 50 years ago Marshall used a pi filter with 8 uF caps. Today I'm looking at a DSL40C schematic and they're using a 22uF and and 8 uF cap in the bias supply pi filter. Contrast that with Fender, who is currently using a single 100uF cap in the DRRI. I know it's not a perfect comparison, but it does show that the big manufacturers are thinking like Chuck -- a little bit is enough, and if that helps to contain cost then that's good enough for the MI market.

getting back to the OP, this thread is about hacking the circuit to improve it, so why not make it more quiet, as long as we don't blow up his amp?

Thanks, yeah, that's the goal: quieter, better longevity, without letting the smoke and sparks out. As much as I tell myself: Ohhh that will NEVER happen, Im REALLY careful, I will always let the amp warm up before I flick the standby. sure as s**t, the other day, I plugged in, flipped the power switch and the standby was off. Bleeahh. And this one has an 80 or 100uf bias cap, and 2 x 22uf first filter caps (44uf). So. Buttons.

[mikepukmel]

I completely agree that larger caps likely will reduce hum. The original design passed the Marshall muster, whatever their standards were.

Its been ages and ages, since the 70's, but I remember, when we were kids, the guys with jobs out of HS had lots of amps, ampeg, fender, marshall, lots of different amps, and SOO many of them had nasty hum, crackle, fizz, problems. No idea how many were due to abuse, and filter caps going bad etc. They weren't quiet amps, for the most part. But, most of the guys would pack 4 guitarists into a garage or basement, with 50 and 100 watt amps, and make so much terrible (did I mention TERRIBLE) noise, that the hum was drowned out during practice, and ears ringing for a week so they wouldn't know a hum if if was there, sadly. In my old age, I have a couple of amps, on a shelf in my TINY office room upstairs, 2 or 3 feet from my head, and a tiny buzz or hum is a killer.

[mikepukmel]

The last thing you need to worry about is tolerances. Remember the stock circuit in your schematic uses 8uf. You already found the OT is not "balanced". 8uf would work, bob's 100uf would work, and anything in between would work. It just isn't critical.

I have slightly different thoughts on relative size of cap. The high voltage caps tend to be smaller, and they charge off the rectifier. Your bias cap only has to charge up to like 50v to be enough to move on, but it is doing that through a 180k resistor. I would have to think that resistor rather than the relative cap size is what slows it down. But then one has to wonder how many seconds does it take the bias to charge up to tube protecting levels, compared to the 10-20 seconds it takes the tube heaters to allow current to flow.

My suggestion to make EVERYTHING adjustable was tongue in cheek. I was using some friendly sarcasm to make the point that making ONE part of the formula precision doesn't change that the whole thing is not remotely precision. It certainly won't hurt the amp to make all the bias adjustments independent. I just think it is wasted energy. But that is only my opinion.

Jokes taken, understood, of course! Thanks, I will leave out the individual bias and focus on getting the rest of the amp parts better (less hum without blowing thinngs UP), better grounding scheme, grid stops in the right places right values to help with oscillation, etc.

[mikepukmel]

I guess it depends on how you define "sufficient," and whether your objective is to repair an amp, build a faithful clone of a classic amp, or design a better amp.

The world is full of noisy amps. I hate that in an amp, and when I'm building I'll do all sorts of little things to try to make noise go away. I'm even willing to do some of those diminishing return things, because I just don't like noisy amps.

The JTM45 schem snippet linked above shows a pi filter, not a single cap filter. A pi filter by it's nature is going to provide far better ripple reduction than a single cap filter, and an 8uF+8uF pi filter is going to provide good ripple reduction. But a pi filter using a pair of 100uF caps rather than a pair of 8 uF caps will have markedly less ripple on it. To see this, just fire up your favorite modeller. the improvement is significant.

Given that caps are so cheap today, I don't see a big downside to going with a larger value. In a market that's not geared to producing low value caps for obsolete tube amps the 100/100 was cheaper from my supplier. That's a win/win scenario for the builder who doesn't feel bound to building to a 50 year old design, just to have authenticity and all of the problems that come with it. I've got nothing against building faithful clones, but at the same time I don't think it's heresy to tweak a classic circuit to improve it.

Hmmm really great question! I think Id like to get 'the tone', but whatever mods are reasonable to help make the amp more quiet and protect the transformers and tubes. Not trying to make an exact replica.

[mikepukmel]

What I'm thinking is that there are many bias supplies that use a single cap. In that case you'd be hard pressed to mitigate ripple on the half wave rectified bias supply even with a 100u cap. With the two cap bias you can do better. The point here is that a bigger single cap probably isn't going filter enough ripple to make it worth while. And besides, we're counting on the output section balance to cancel most of any hum anyway. And Mike's intent here is to achieve good balance which will only improve that. So the cap value really only needs to be big enough to filter gross ripple and decouple LF below audio to avoid any LF signal interaction via the grid loads where they meet atop the bias supply IMPEDANCE. 8u is plenty for that. There was a guy here for a while (SN rhymes with moundsurugan) who insisted again and again that increasing the bias cap value improved low end. Hogwash. I think there are a couple of Mesa designs where this may marginally be true, but not at all for 90+% of guitar amps.

I guess my point is that a single 100u cap won't filter the ripple from a typical bias supply anyway and two would stretch the charge time to where it may need to be considered. 8uf is big enough to decouple. I say leave the supply design alone. It works well like it is.

How cool is that! Aaaand the kit comes with 2 x 8uf's!

[bob p]

A friend of mine from out of state scheduled a visit to use my house as a base to do some HiFi shopping. He wanted to spend a ridiculous amount money on a Sonus Faber speakers, and the closest audio shop that carried them was in a Chicago suburb. The boutique audio store was set up like a living room, and they had an assortment of amps that they would hook up for your listening pleasure. What really amazed me was that there was a very expensive Hovland EL34 amplifier that buzzed like a beehive when the store owner hooked it up. My friend was able to ignore the buzz, but it drove me crazy. I couldn't understand how any piece of gear could be considered High End when it buzzed like a 50 year old amp that needed a recap. I guess it bothers some people more than others.

[bob p]

Thanks, yeah, that's the goal: quieter, better longevity, without letting the smoke and sparks out. As much as I tell myself: Ohhh that will NEVER happen, Im REALLY careful, I will always let the amp warm up before I flick the standby. sure as s**t, the other day, I plugged in, flipped the power switch and the standby was off. Bleeahh. And this one has an 80 or 100uf bias cap, and 2 x 22uf first filter caps (44uf). So. Buttons.

There are two ways to design an amp: one is to design everything with very conservative values so that nothing bad happens when you don't use the standby, and the other is to design an amp with other considerations being more important, which ends up requiring use of the standby. Personally, I don't use the standby all that much. I think the considerations about cathode stripping are over-blown. In a design that really needs a pre-standby warmup, I think it's a good idea to put in timer controlled relay.

[mikepukmel]

A friend of mine from out of state scheduled a visit to use my house as a base to do some HiFi shopping. He wanted to spend a ridiculous amount money on a Sonus Faber speakers, and the closest audio shop that carried them was in a Chicago suburb. The boutique audio store was set up like a living room, and they had an assortment of amps that they would hook up for your listening pleasure. What really amazed me was that there was a very expensive Hovland EL34 amplifier that buzzed like a beehive when the store owner hooked it up. My friend was able to ignore the buzz, but it drove me crazy. I couldn't understand how any piece of gear could be considered High End when it buzzed like a 50 year old amp that needed a recap. I guess it bothers some people more than others.

wow, no kidding and I always had the idea that tube audiophiles were way pickier about buzzing and hum than I am. Im with you, I couldn't stand it especially from an audio amp that you'd be listening to music with. At least with guitar amps, if you play in a band, and its not a studio amp, some of that will get masked.

[mikepukmel]

So, back to this amp, the really nice looking readable schematic dated 5/12/01 and a little less readable one that says "REISSUE" dated 1988, neither have grid stops on the output tubes, or the second gain stage 12ax7 tube.

I see a bunch of late builds where they take the 100k plate resistor, mount it right on the tube pins (on the tube that has the cathode folloer), then bend one of the leads over to the grid of the second half of the tube. No grid stop.

So, wondering: is it ok to put 5.6k grid stops on the output tubes like some later models had, and also put grid stops on the second preamp tube. Surprised to find quite a few 60's Marshall schematics with no grid stops except on the first preamp tube.

The thread has had some great discussions about upping the filter caps on the bias supply, seems like its probably OK not to go over 32uf (size of first filter cap that the GZ34 sees) minus some tolerance cushion. But I already have the caps, so might build it and swap those out if the amp is too noisy. Easy to do, where they are located.

Grounding schemes have been gone over extremely well, and I have copies of the MEF pages in case I have to go without internet for a while!

Solid state rectifiers "immortal mod" on the GZ34 for soft fail. (tubes ain't what they used to be).

Shielded wire on the input wiring.

Nothing soldered to the back of pots, use the modern grounding schemes.

The Valvestorm kit and layout has huge 5W 470's on the output tube grid2, but they don't have the 1k 2w that all of the other builds have between where the 2 470R resistors tie together and the power rail (after the choke). Doesn't look "right" since all the other builds and schematics have this resistor. "They wouldnta put it there if it wattnd neededed"

The schematic from '01 has one fuse on the PT HV center tap. Immortal says to instead put one on each HV leg. Plenty of room there, and I have the parts.

After the thread discussion, probably won't bother with individual bias feed for each output tube. But I would like to find a fender like pot that I can mount to the chassis, the one that comes with the kit is a cheesy little thing, very flimsy looking. I would not trust it in a guitar amp that is going to get lots of vibration. The only ones I could find were 10k fender spec. Still looking.


A really skilled builder that posted his build on MEF mounted the 1R bias check resistors between 3 panel mount pin plugs on the back of the amp, then ran a very short wire to the tube socket. Looks really nice, and easy to check bias without taking the amp apart.

Can't think of anything else.