What have I done? A little of everything.

[Pierre]

45 watt project.bmp

So, I've been thinking about this one for a long time and started the project almost as long ago. Right now, the amp is fairly quiet and stable even with everything maxed and sounds pretty good. This is my fourth build and the goal was to try a bunch of stuff that I have seen on this site and others and that I hadn't tried before.
My previous builds have been < 300v B+ lower wattage builds, so I wanted to go bigger and try el34s. My previous builds were cathode biased and I wanted to go fixed biased, include a parallel effects loop, and have some relay switching. I did include a couple of elements from my last build which are a 5879 in V2 position and a choke before the power tube plate supply.
Here's the problem, I feel like I built a dragster and I can only test it on the street. This thing is loud! My goal with the relays was to have the ability to have two completely isolated channels and also be able to combine them in parallel(how should I label that?). The third relay adds a cold clipper as a boost. I'm looking for advice to quiet the relay switching and opinions on voicing the amp.
There's too much of everything. The bass is huge and boomy and there's a ton of highs. I've reduced some of the coupling and bypass caps and added the mix resistors after the first stage so that they work properly in parallel. I'm avoiding lowering the coupling caps on the power section because I don't want to lose anything.
Unfortunately, I'm not a guitarist, I just love music and tinkering. I may just need to get this thing with a player in a room that can handle the volume for a proper test. If anyone would care to look over the schematic and offer suggestions, I'd appreciate it. The power supply is unusual, but it is what it is. I'll try to come up with some photos, but all of my careful initial work went out the window once the amp was powered up. Thanks to everyone for this site and the great info. I visit other amp sites, but this is always my "go to" site.
Pierre

[10thTx]

Thanks for sharing your work and the schematic!

With respect, 10thtx

[xtian]

Great job on the schematic. It’s remarkable that you’ve taken on such a large number of variations, and that you’ve got a working amp!

You’ve got a PPIMV, so I’m curious why you find the amp so loud and overwhelming?

The schematic shows 470K resistors in the power section, usually used as cap balancing resistors, but no connection to the junction between the stacked caps is shown. If this is true in reality, you’ve just got bleeder resistors, but no balancing happening.

What cab/speaker(s) are you playing thru?

[martin manning]

You seem to be missing a couple of connections at the middle of the B+0 and B+1 stacked filter caps, and the balance resistors on the B+2 stack. Also, what is the purpose of the extra diode and 220k resistor in the bias supply?

[Pierre]

Thanks for noticing the missing connections in the schematic. They are physically there in the amp. The bias supply is borrowed straight out of Merlin's book and from my understanding of the text the extra resistor is a safety in case the pot wiper ever failed. I'm not an EE so I rely on the work and help of others. I will fix the balancing resistors on the schematic.
My 2 concerns at this point are lessening the pop on the relays and voicing the amp. My current thoughts are of keeping the coupling caps smaller on the front end but leaving them larger downstream. I've been playing with LED's on the cathodes of the preamp tubes which eliminates partial bypassing but I'd love to hear people's opinions on the merits of adjusting bypass caps vs coupling caps along with their relative positions upstream or downstream in the signal chain.
Also, does anyone have a strong opinion on voicing the cold clipping stage? Right now the way the amp is configured, the cold clipper really doesn't feel like it's adding volume but instead maybe changing the tone and feel? I have a small coupling cap on the output of the stage, would bigger be better? I originally fed this stage from the same power supply node as the effects loop, but felt that perhaps I needed more isolation between that stage and the effects loop stage and added a resistor and filter cap.
Again, this amp is stable with everything dimed. However, I'm thinking I need to dump some gain somewhere to make things more user friendly? The PPMV definitely works to cut the volume but changes the character of the amp. I built an air brake for the last amp which I gave to my nephew along with the amp. I have parts to build another air brake for this one. This has been a thought and design experiment and I appreciate the feedback from everyone.

[statorvane]

My 2 concerns at this point are lessening the pop on the relays and voicing the amp.

Can't help with the relays, but your power rail filtering seems really low from B+4 and after (22 & 10 uF). If the bottom end seems too flabby/boomy , try upping all those to at least 33 uF and see how that feels.

[martin manning]

Thanks for noticing the missing connections in the schematic. They are physically there in the amp. The bias supply is borrowed straight out of Merlin's book and from my understanding of the text the extra resistor is a safety in case the pot wiper ever failed.

Do you have balancing resistors on the B+2 filter? If not you should add them. I can understand the safety resistor on the bias pot, but not the diode as shown. Since it's working, maybe you haven't drawn the bias supply as-built?

How's the Presonance working? You said the amp is stable all the way up, so I guess it's doing fine. Don't forget that PPIMV will effectively alter the feedback ratio, which will change the sound and break-up characteristics. You might try selecting a different OT secondary tap for different MV settings.

I'll study the channel relays more later.

[Pierre]

The bias supply is built as drawn. I'll take another look in Merlin's book to refresh my memory on the extra diode. I can easily add the balance resistors at B+2. To be honest, at that stage in the supply I didn't think I'd need them.

As far as the later stage filtering, I wanted to try the lower cap values and see how it affected the feel. The bass isn't flabby but if you even touch the low E string on the guitar, it goes into feedback. That's at only around 8:00 on the V1A volume with single coils as well as humbuckers. Would that be caused by lower filter values? I guess what I'd like is a little more balance on the fretboard as far as the sustain is concerned. It could very well be the speaker cab too which is a small 2x12 partially open back cab.

I'm sure if I lower the coupling cap from .02 to .01 that will help cut some bass but I was hoping to have V1B as more of a bright/modern channel and V1A as a more vintage sounding channel for contrast. I may just have to lower the gain on V2 to balance things out.

The presonance is very stable and I can use the cut control to trim off some highs.

I'd love to hear what others may have tried in the process of fine tuning their builds. When I look at the old Fender schematics it's hard to believe what they used for coupling/bypass/ filter cap values. The V1B channel >V2>cathode follower topology is a carbon copy from the preamp on my previous build which worked well. That amp had much lower B+ and a TW rocket PI into 2xEL84 power section.
Thanks again for all of your thoughts on this.

[Pierre]

I just read the thread on detuned cabinets. I heard descriptions of wall shaking bass caused by speaker/cabinet problems. I think that's where I need to start looking.

[brewdude]

You might try reducing the screen bypass cap on the preamp pentode—that might take some bass. Perhaps, .1uF or .05uF instead of .2uF(?).

[brewdude]

I’m curious about haw the loop is working for you. Have you tested it and all it’s functions (phase switch, send level and blend)? Have you tried bepassing the loop all together?

[brewdude]

I’m also curious about your CLC pi filter. What are the spec’s of your choke?

I have also played with some CLC filters and true choke input power supplies. I wonder if you have too much filtering (if that’s possible). I found in some of my builds that less capacitance up front sounded better, where too much made the amp too stiff and sterile.

[brewdude]

Maybe a more traditional pre phase inverted master volume would be beneficial.

[Pierre]

The loop seems to be working well. I have only had a 15' instrument cable in the loop so far and a crappy autowah because that's what I have around and all functions appear quiet and stable. I screwed up and put in an audio pot for the mix control so there's a bit of a hump but it's staying for now.

I used the CLC filter in my last build and it worked well, although I don't have anything to compare it to. I'm away from home, but the choke that I used was a Hammond open frame choke - I think 159T?

With the boost stage engaged, the entire preamp goes through that pot. Also, the fx mix can act as a crude volume with nothing in the loop.

I think I'm going to try a different speaker/cab next. I built a nice little closed back 1x12 with an eminence delta 12 for my nephew based on plans and recommendations from this site. I'll see if that handles all frequencies better.

Thanks for your thoughts on the filtering and screen bypass. I'm avoiding the downstream filter caps because they're a pain to get to. I may play with the screen on the pentode to see if I can change anything for the better. I never thought about partially bypassing the screen.

[Pierre]

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I can understand the safety resistor on the bias pot, but not the diode as shown. Since it's working, maybe you haven't drawn the bias supply as-built?

Martin,
I looked back at Merlin's book and the extra diode in the bias supply is to slow down the discharge time for the second filter cap on the bias supply. Apparently, this would help if the amp were switched off and back on quickly. Not sure if it's necessary, but diodes are cheap!

Maybe a more traditional pre phase inverted master volume would be beneficial.

I played with using the mix control as a pre PI master and it worked great! Thanks for mentioning that.

I’m also curious about your CLC pi filter. What are the spec’s of your choke?

Confirmed. Hammond 159T

I reduced the cathode bypass cap on V1A and that took some of the boominess away. A different speaker cab should fix the rest. I increased the coupling cap on the cold clipper stage as well to see what effect, if any, it would have. Nothing dramatic, so I'll leave it alone. Other than that, I think this amp is close to where it needs to be. Next step, quieting the channel switching and deciding what to label the controls.