Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mountainhick on January 19, 2026, 03:01:14 pm
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I've had mixed results with these little beasties and have induced redplating and runaway. The scuttlebutt is 250 plate volt limit, but i see PP schematics kind of all over the place. I have yet to find a guitar amp SE schematic.
What do y'all know, what are your favorite 6AQ5 configs (I've seen Geezers quad, looking for info for just SE and PP not PPP).
Thanks
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My favorite is https://sluckeyamps.com/smoky/smoky.htm
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My favorite is https://sluckeyamps.com/smoky/smoky.htm
Thanks! That's interesting and looks right on max dissipation. One of my previous PP projects redplated at less dissipation. Curious.
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had mixed results also til I modified the PA a 'lil
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had mixed results also til I modified the PA a 'lil
Uh? 6AQ5 to KT88?
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I have yet to find a guitar amp SE schematic.
Univox U-41/U-42/U-45
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I have yet to find a guitar amp SE schematic.
Univox U-41/U-42/U-45
That one is more on target in terms of my experience running 6AQ5.
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looking for info for just SE and PP
Univox made 3 PPs: the U-72/U-75, U-102, and the U-155R
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Here are two. Just posted the first in the other thread, but this seems a good place to collect them.
1 - MIJ single ended amp with single diode rectifier, 12AX7, and 6AQ5
2 - A Champ-type amp schematic by Adam Alpern that I found elsewhere, with diode rectifier, 12AX7, and 6AQ5. I built an amp very similar to this but haven't done a complete schematic yet - I used a toroidal transformer, added a switchable tone control, switchable V1a bypass cathode, and a pot for variable NFB. This one sounds particularly good with a strat.
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Here are two. Just posted the first in the other thread, but this seems a good place to collect them.
1 - MIJ single ended amp with single diode rectifier, 12AX7, and 6AQ5
2 - A Champ-type amp schematic by Adam Alpern that I found elsewhere, with diode rectifier, 12AX7, and 6AQ5. I built an amp very similar to this but haven't done a complete schematic yet - I used a toroidal transformer, added a switchable tone control, switchable V1a bypass cathode, and a pot for variable NFB. This one sounds particularly good with a strat.
That Micro Champ is just about spot on what I've been thinking for a happy SE 6AQ5
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They worked very well in this amp
THE HOWLER (https://el34world.com/Forum/index.php?topic=33240.msg369323#msg369323)
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They worked very well in this amp
THE HOWLER (https://el34world.com/Forum/index.php?topic=33240.msg369323#msg369323)
Not seeing any schematic?
I have a howler monkey schematic I found I while ago is it the same as this?
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Also note that the 6AQ5 is listed as 250V max, but from what I've dug up from other peoples' projects, it's safe to go up between 250-300V.
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On the PP side. I have an Epiphone EA50T (schematic is in Doug's Library) I modded the preamp to remove some filtering, but the power amps is stock 2x6AQ5. As you noted the B+ is a bit below 250. It's one of my favorites.
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Uh? 6AQ5 to KT88?
:laugh:
rev .101 was a PSE 6AQ, just never got where I wanted, thought about up'n it to 4, had a bench KT88 n iron on hand, so.....
from there I never looked back at the 1/2 dozen small-bottles i'd already tinkered with.
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Also note that the 6AQ5 is listed as 250V max, but from what I've dug up from other peoples' projects, it's safe to go up between 250-300V.
My query pertains directly to this issue. Of course there are many ways to skin the cat, but I did have one project that ate/redplated tubes at 280 plate volts. Sometimes schematics can give up pearls of wisdom regarding B+, biasing and screen control that allow lower dissipation at higher voltage. I m pretty surprised at the Sluckey SE shared above, and expect the lower B+ amps you cited will provide longer tube life.
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I suspect that "safe" operation does not really equate to tube longevity all that well. Back in the day - carry spare tubes. No crying when a power tube goes. With the price of tubes today it makes sense to either have a lot of amps, so that none get stressed too much, and/or use power tubes within, or close to, their design specs. But then I prefer amps that do not push the power tubes too hard. I may be in the minority there. But 6V6s run at 325 to 350 Vs are sweet.
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I suspect that "safe" operation does not really equate to tube longevity all that well. Back in the day - carry spare tubes. No crying when a power tube goes. With the price of tubes today it makes sense to either have a lot of amps, so that none get stressed too much, and/or use power tubes within, or close to, their design specs. But then I prefer amps that do not push the power tubes too hard. I may be in the minority there. But 6V6s run at 325 to 350 Vs are sweet.
I do believe that operating within spec promotes longevity, and it remains a serious consideration for me as well. The other aspect is the option to choose readily available cheaper tube types, but that too has it's own idiosyncrasies.
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The other aspect is the option to choose readily available cheaper tube types, but that too has it's own idiosyncrasies.
Yes it does. I have had fun and success with 6973 alternatives 6CM6 and 6CZ5.
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but I did have one project that ate/redplated tubes at 280 plate volts
Plate voltage alone does not cause red plating. With the proper screen voltage control (you must protect the screens from over dissipation) and bias, they will not red plate. I believe a plate voltage of 280V is not an issue for the 6AQ5 and also that there is room to push the plate voltage even a bit further. All three of the PP amps I posted all run plates over 250V with the U-155R over 300V. IMO, unless you push the voltage so high that the tube arcs, plate voltage above 250V is not a problem.
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The other aspect is the option to choose readily available cheaper tube types, but that too has it's own idiosyncrasies.
Yes it does. I have had fun and success with 6973 alternatives 6CM6 and 6CZ5.
I have some 6CM6 and also 6CL6 on deck. Have used 6AK6, 6F3P, 6F4P, 6P1P
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but I did have one project that ate/redplated tubes at 280 plate volts
Plate voltage alone does not cause red plating. With the proper screen voltage control (you must protect the screens from over dissipation) and bias, they will not red plate. I believe a plate voltage of 280V is not an issue for the 6AQ5 and also that there is room to push the plate voltage even a bit further. All three of the PP amps I posted all run plates over 250V with the U-155R over 300V. IMO, unless you push the voltage so high that the tube arcs, plate voltage above 250V is not a problem.
Of course. Revisiting that project I might have upped screen resistor values, and they may have been weak tubes though they tested well on my SECO.
That project though, the old scrounged PT was able to provide 300V B+ to a pair of EL84, so swapped and never looked back.
I also found posts by Geezer here that talked about having to reduce dissipation until he had redplating in control. Seems not an entirely inconsistent problem. I remain impressed by Sluckey's SE version.
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but I did have one project that ate/redplated tubes at 280 plate volts
Plate voltage alone does not cause red plating. With the proper screen voltage control (you must protect the screens from over dissipation) and bias, they will not red plate. I believe a plate voltage of 280V is not an issue for the 6AQ5 and also that there is room to push the plate voltage even a bit further. All three of the PP amps I posted all run plates over 250V with the U-155R over 300V. IMO, unless you push the voltage so high that the tube arcs, plate voltage above 250V is not a problem.
I'll add that looking over the schematics shared to this thread. there's a marked difference in the versions designed closer to tube spec sheet norms compared to the screaming banshees! And I know for my stash of 6AQ5s how I will approach things.
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I am perplexed by Sluckey's "Smoky" running higher screen voltage than the plate. Is there anything with the large choke that would help regulate this?
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SEL49 knows Sluckey really well :icon_biggrin: So I imagine he will jump back in.
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Seems that the Fender Musicmaster 6AQ5 wasn't listed
Here the schematic and some info from Tubestore
https://www.thetubestore.com/fender-musicmaster-6aq5-schematic (https://www.thetubestore.com/fender-musicmaster-6aq5-schematic)
Franco
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Oddly enough, my first ever amp (way back in high school) was a Fender Musicmaster Bass amp - it was most likely the 6AQ5 version. I had no clue at all about amps, tubes or anything, just that I needed an amp for my bass (my first guitar of any kind). I ended up selling the amp after a few years because I needed something louder (got a Peavey TKO). Seeing the Musicmaster amp has gone up in value, I wish I'd kept it.
It appears the Musicmaster has an interstage transformer for the phase inverter - anybody know why they did that rather than use a tube? Could you easily substitute a tube PI there?
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in the mid 20th century iron was cheap, like Leo, less parts, less cost, more $$'s
I built 2 using I.T. for the PI, one got a round of thumbs up from the guitarists
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Oops, wrong thread.