Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: captaintrav on November 08, 2010, 02:43:47 pm
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I just bought back my old Bassman 10 from a buddy of mine, I only sold it when I was going through one of my "phases", anyhow, I couldn't remember how I left it, so I pulled it apart, and this time, I'm going to write this down.
Disclaimer: I don't claim to know what I'm doing, so if I screwed something up here, let me know in no uncertain terms, this is just a hobby of mine (obsession more like it)!
The amp has worked flawlessly for probably 6+ years like this, but I think I can do better with it, once it starts to break up it sounds too harsh.
Filter caps, OT and speakers are replaced. Stupid 32ohm speakers gone, 8ohm speakers now in parallel/series wiring. Stupid bias supply fixed to be useful. Stupid tonestack on "Normal" channel changed to more of a typical Fender style. Replaced the huge resistor with a choke. Amp hardly hisses or hums at all...puts every PCB amp I've tried to shame in this regard.
I attempted to "Blackface" PI-Output, which sounded not great, lots of "farting out" I guess. After finding this: http://www.aikenamps.com/BlockingDistortion.html, I made a few changes. That's why bias supply resistors are back to 100k instead of the BF value of 220k, and coupling caps are .047 instead of .1. Helped the farting out a fair bit, but the amp sounds sterile. Luckily this was not one of the later models where they were doing crazy things to get more and more power out of a 2x6L6 amp, but B+ is around 500v. Some people have suggested a good mod is to go tube rect, but I'm not looking for a tweed sound here, and tweeds didn't have 500v B+ either, I'm thinking of just changing the PT.
Here's a schematic I drew on where I hacked it up: http://img.photobucket.com/albums/v258/axeman2k4/Bassman_10_alt.jpg
Some thoughts:
1) Larger cathode bypass on the second gain stage. Both channels share the cathode resistor, is there any rules about changing the bypass cap size in this scenario?
2) Extra coupling caps in a few places, turf them? there is a .0047uf before the volume control on the "studio" channel. Seems unusual, but the amp has a ton of low end it seems, I'm guessing this has to do with the closed back cab with the "ports" in the front. It seems to me I plugged these at one time, but even then it seems pretty bassy. The "extra" coupling caps I'm referring too are the ones that are in spots you wouldn't expect in a typical BF Fender preamp stage.
3) Replace the PT, lower B+ could make for sooner breakup, and less farting out, maybe properly "BF" the amp and get good results this time?
4) turf the MV. I would already, but I don't know how much it affects the tone - I leave it at 10 all the time anyhow, and I'd have an extra ugly hole in the front panel. If the dimensions are right, I could find a repro BF bassman piece to go on the front.
5) add an extra 12AX7 to make the normal channel more like a BF bassman? Is that worth it, seems a fair bit of hacking?
6) I have a couple of NOS Phillips 7581 (I think) tubes I pulled out of a fake (PCB) Super Reverb with toasted screens. Should I try using these, I don't know what to bias them at or anything... maybe the toasted screens were b/c of these ? I replaced the screens with 3watters.
7) change something else?
PS. Is there any reason to use 1watt resistors on all of the plates, looks like this was spec'ed on the schematic for this amp, but not on later fenders. For some reason I have those even on the bias feed resistors, am I just wasting money here?
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I would change the silly MV for a PPIMV or a bootstrapped PI MV, reduce overall NFB as well as PI gain and play with coupling caps.
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Hi captaintrav,
What do you mean by " turf" them?
Thanks, Brad
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What do you mean by " turf" them?
Remove.
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Thanks FYL.
Brad
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I would change the silly MV for a PPIMV or a bootstrapped PI MV, reduce overall NFB as well as PI gain and play with coupling caps.
I'm looking at some schematics, I found one of a Matchless that just has a 1M pot between the PI outputs, just after the coupling caps. Is this a typical PPIMV? Seems pretty easy, no caveats or beware dragons doing this? It might actually do something interesting here, the MV at the PI input is just silly.
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Mtchless uses a crossline, which is fine with cathode-biased amps. The BM uses fixed bias, you'll get much better results using a bootstrapped-PI MV, a Lar-Mar or type II PPIMV.
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The types of master volumes FYL is referring to are found in the ARCHIVES:
http://www.el34world.com/Forum/index.php?topic=3496.0
With respect, Tubenit
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Thanks, it did occur to me that the "crossline" style MV might be a bad idea on a negative grid biased amp. Figures it'd be two easy to use the existing MV pot for anything useful.
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Disclaimer: I don't claim to know what I'm doing, so if I screwed something up here, let me know in no uncertain terms, this is just a hobby of mine (obsession more like it)!
I should put this as the heading on every one of my posts. But, I usually learn the hard way by screwing it up the 1st and asking questions later. I am trying to change my ways. LOL
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Figures it'd be two easy to use the existing MV pot for anything useful.
You may use it as a bootstrapped PIMV.
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OK, so I haven't touched the MV yet, but after biasing the tubes something occurred to me, since the B+ is 500v or more. So I checked the plate voltage elsewhere, the plate voltage on the first two gain stages was 300v or more, I'm guessing this doesn't help the amp sound any less "harsh".
In my brilliant ignorance, I had changed most of the supply to blackface values, 4.7k and 1k, resistors + choke, instead of 10k, 2k, and a 2k 7watt resistor... the change to the choke alone probably means less voltage drop than the old amp... so I'm getting lots more volts on the PI and preamp stages than even the stock silverface circuit...duh....
So since the PT has the 5v secondary, I wired up a 5AR4, more because I knew that it would drop the B+ than looking for it to change the sound. This helped alot, I think.
I think next I'll go back to 10k and 2k voltage divider resistors in the supply. Good idea, or even replace the PT with a "proper" one? - the I get something like 373VAC on each leg of the secondary. The amp sounds best to me biased on the cold side, but doesn't break up that easy, perhaps the high B+ is a factor here?
Another general question. Any idea why from the factory there are two wires connected to ground on the PT. One's the CT for the HV secondary, but the other is a yellow wire with a red stripe. It's not a CT for the 5v secondary, checked that with a meter, it reads like something in the megaohms to any other wire?
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Another general question. Any idea why from the factory there are two wires connected to ground on the PT. One's the CT for the HV secondary, but the other is a yellow wire with a red stripe. It's not a CT for the 5v secondary, checked that with a meter, it reads like something in the megaohms to any other wire?
Heater CT? (if so, it should have continuity to the rest of the heater winding), or (faraday) shield? (Just guessing)
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Maybe, bias tap? See if theres a voltage on it. 50 to 100 Vac?
Brad :smiley: