From my hodgepodge of parts I've got
a pair of TT-119-OT 15 watt 5K SE transformers
a 550V (275-0-275) at 175mA center tapped, 6.3V at 4A and 5V at 3A transformer
and a pile of 6V6GT, 6L6GC, a couple EL34, KT66 and 6550 tubes
a 12" x 8" x 3" chassis.
What would you build with that? I've run out of room for amps so it will likely be my last build for quite some time.
It would likely go in a small room playing at fairly low levels.
I have a kegger/blueglow SE amp that I'm using now in another room running 6L6GC tubes, and I do like it but would like to try something different..
a pair of TT-119-OT 15 watt 5K SE transformers
a 550V (275-0-275) at 175mA center tapped, 6.3V at 4A and 5V at 3A transformer
and a pile of 6V6GT, 6L6GC, a couple EL34, KT66 and 6550 tubes
a 12" x 8" x 3" chassis.
What would you build with that? I've run out of room for amps so it will likely be my last build for quite some time.
It would likely go in a small room playing at fairly low levels.
I have a kegger/blueglow SE amp that I'm using now in another room running 6L6GC tubes, and I do like it but would like to try something different..
6V6 seems like the best match with the power transformer.
Maybe 6V6 SET? A bit under 2W. Not a great use of the OPTs, though. They wouldn't be your bottleneck, anyway. 🙂
Maybe 6V6 SET? A bit under 2W. Not a great use of the OPTs, though. They wouldn't be your bottleneck, anyway. 🙂
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The O/P iron will do nicely for full pentode mode 6V6s. The primary impedance is quite acceptable and the magnetic headroom needed to support the requisite loop NFB is present.
(275VRMS) (21/2) = 389VDC. That's a bit "tall" for 6V6s. So, I suggest you rectify the B+ with a 5R4 and take advantage of its large forward drop.
Regulate g2 B+ with a 0A2 + 0B2 gas regulator stack.
A 12AX7 or 6SL7 will provide the needed pair of voltage amplifiers.
(275VRMS) (21/2) = 389VDC. That's a bit "tall" for 6V6s. So, I suggest you rectify the B+ with a 5R4 and take advantage of its large forward drop.
Regulate g2 B+ with a 0A2 + 0B2 gas regulator stack.
A 12AX7 or 6SL7 will provide the needed pair of voltage amplifiers.
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You've got enough 5V current for a 5U4(G) or 274 rectifier, which will drop that B+ even more if you want, and they do add some visual flare 🙂
Forward drop in 5R4s is greater than that in 5U4s. Chinese and Russian ST bottle 5U4 "equivalents" are GARBAGE, while U.S. made 5U4Gs are silly expensive. A ST bottle 5R4 can be sourced for "reasonable" $.
You might try using 7591 tubes. The have a little more transconductance than a 6V6. The 7591 works better with higher screen voltage, and higher plate voltage than what a 6V6 likes to operate at.
The 5k transformers have a 40% tap.
Reverse the plate and B+ primary connections, and connect the screen to the UL tap.
That is 60% "UL" (not real UL mode) but it is 60% of the way to Triode wired mode.
Should be a good tradeoff.
Just an easy way to wire an output circuit, and UL has some negative feedback.
And, a 7591 is easy to drive, lots of driver tubes can easily drive it. It will also allow for cool operation of the power transformer.
The other tubes (except 6V6) you mentioned wii require more filament current, and more plate current, to operate as optimum as possible with that combination of parts.
Those look like real nice transformers. At first I looked at the wrong ones.
But the 5K has 60H and 100mA rating.
That is easy for an SE 7591 in UL (40% or 60%) to drive.
The 5k transformers have a 40% tap.
Reverse the plate and B+ primary connections, and connect the screen to the UL tap.
That is 60% "UL" (not real UL mode) but it is 60% of the way to Triode wired mode.
Should be a good tradeoff.
Just an easy way to wire an output circuit, and UL has some negative feedback.
And, a 7591 is easy to drive, lots of driver tubes can easily drive it. It will also allow for cool operation of the power transformer.
The other tubes (except 6V6) you mentioned wii require more filament current, and more plate current, to operate as optimum as possible with that combination of parts.
Those look like real nice transformers. At first I looked at the wrong ones.
But the 5K has 60H and 100mA rating.
That is easy for an SE 7591 in UL (40% or 60%) to drive.
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The PT is an excellent match for the OTs. 275VAC makes 385VDC. 385VDC in 5K is 77mA. Two 77mA loads is safely inside PT's current rating.
6V6 won't make over 6W, won't "fill up" a 15W transformer. Sure, it can work fine. But not "best use" of iron in hand.
To fully use a "15 Watt" OT you need >30W of plate dissipation. But the PT is just good for 30W*2. So 30W it is. More than 6L6 or EL34. (I do not trust 6L6GC at steady 30W.) This is 6550/KT88 turf. Or add more drop in the filter (you need it) and settle for 25W Pdiss 10W output.
There's hardly anything wrong with the Fender AA-Champ plan, only scaled-up to higher final power.
6V6 won't make over 6W, won't "fill up" a 15W transformer. Sure, it can work fine. But not "best use" of iron in hand.
To fully use a "15 Watt" OT you need >30W of plate dissipation. But the PT is just good for 30W*2. So 30W it is. More than 6L6 or EL34. (I do not trust 6L6GC at steady 30W.) This is 6550/KT88 turf. Or add more drop in the filter (you need it) and settle for 25W Pdiss 10W output.
There's hardly anything wrong with the Fender AA-Champ plan, only scaled-up to higher final power.
Small signal circuitry that works with 6V6s and EL84s also works with the 7591. Fisher, Scott, Sherwood, and L_RD knows who else exploited that fact.
The power trafo's rectifier winding is a bit "short" for 7591s. So, SS rectify the B+ and 1/2 wave parallel voltage multiply the 5 VAC winding to obtain a C- supply, for "fixed" bias. The multiplier schematic is for a + rail; so, reverse the polarity of the diodes.
In order to keep voltage amplifier gain up and also comply with 7591 grid to ground resistance limits, buffer the high μ/high RP triode with a ZVN0545A source follower DC coupled to the triode's plate.
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I didn't see 7591s, 6550s or KT-88s in the OP's list of parts, and I just checked again. Nope. Not there.
He clearly asked what to build out of the parts listed.
He clearly asked what to build out of the parts listed.
"Regulate g2 B+ with a 0A2 + 0B2 gas regulator stack."
That is a bit above my understanding level.
To loosen the constraints a bit if I had to spend $100 on new tubes, PT or other parts it wouldn't be a deal breaker.
I just found another power transformers in my stash:
850VCT 120 mA
5V 3A
6.3V 2.5A
That is a bit above my understanding level.
To loosen the constraints a bit if I had to spend $100 on new tubes, PT or other parts it wouldn't be a deal breaker.
I just found another power transformers in my stash:
850VCT 120 mA
5V 3A
6.3V 2.5A
All of your output valves will plug into the same 7AC octal socket (connect pins 1 and 8 to include the EL34), and you have (just) enough heater power available for any of them plus any common 5T octal socket (take output from pin 8) twin rectifier. Wire the whole thing up triode connected, with a nice twin triode driver (or better) and Bob's yer uncle Bob.
My point being, you don't have to limit yourself to anything other than a driver stage valve socket at this point. Might even permanently mount several choices of output stage cathode resistors for easy "tube rolling". Or an adjustable "fixed" bias. But always, always make it easy to measure bias current.
A coupla-Watt stereo amplifier is lifetime useful.
YOS,
Chris
My point being, you don't have to limit yourself to anything other than a driver stage valve socket at this point. Might even permanently mount several choices of output stage cathode resistors for easy "tube rolling". Or an adjustable "fixed" bias. But always, always make it easy to measure bias current.
A coupla-Watt stereo amplifier is lifetime useful.
YOS,
Chris
Open loop linearity in full pentode mode is maximized, when screen grid (g2) B+ is regulated at some fraction of plate B+. In particular, highly irritating intermodulation distortion (IMD) is held down. The 0A2 is 150 V. and the 0B2 is 108 V. The linked 0A2 data sheet shows how to wire gas discharge regulators in series. A 0A2 plus a 0B2 gets you a nice 258 V. for 6V6 screen grids.
Ignoring losses, rectifying and cap. I/P filtering 425 VRMS yields (sic) 601 VDC, which is way too high. The 425-0-425 power trafo should (IMO) be 5U4GB rectified and choke I/P filtered. A rough estimate for the resulting rail is 360 VDC. Unfortunately, given the "beating" it will take, a 10 H. choke up to the task will set you back several $. Even with the requisite derating, Hammond's nominally 300 mA. 193M should get the job done, but it weighs 10.5 pounds. Heyboer might be able to wind you a suitable choke I/P rated inductor at lesser expense.
Be careful with 'plug and play'.
The 7591 has a different pinout than all the other tubes that are mentioned.
Ignoring that would be 'plug and pray', and get the fire extinguisher out.
And since the EL34 has now been mentioned, don't forget the KT77, it has very close specs to the EL34.
With a wire from pin 1 to 8, that is a 'plug and play' combo.
The OP said small room and fairly low levels.
Let's not design a 30 Watt amp, unless he has in-efficient speakers.
Did I miss something, is this for a Hi Fi amplifier, or is it for a Guitar Amp?
Please, posters, mention it up front if it is for a Guitar amp.
Guitar amps are in the Instruments & Amps threads.
Some of us do not know what a kegger/blueglow SE amp is (maybe I am the only one who does not know).
The 7591 has a different pinout than all the other tubes that are mentioned.
Ignoring that would be 'plug and pray', and get the fire extinguisher out.
And since the EL34 has now been mentioned, don't forget the KT77, it has very close specs to the EL34.
With a wire from pin 1 to 8, that is a 'plug and play' combo.
The OP said small room and fairly low levels.
Let's not design a 30 Watt amp, unless he has in-efficient speakers.
Did I miss something, is this for a Hi Fi amplifier, or is it for a Guitar Amp?
Please, posters, mention it up front if it is for a Guitar amp.
Guitar amps are in the Instruments & Amps threads.
Some of us do not know what a kegger/blueglow SE amp is (maybe I am the only one who does not know).
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7591 was in my mind as well when I read this. This could be used in triode and still give enough output for a small system. Or UL as stated. And given the higher mu it would work with a nice beefy driver like EL84 in triode, which also has a higher mu around 20. So a simple 2-stage amplifier.
I've been using a very similar topology with EL33 into EL12n and that sounds pretty good. These are European tubes, though, and 7591 is more common stateside. When I replaced the EL12n with an EL34 in triode the sound wasn't so good, though it wasn't a premium pair and they were fairly used. And as stated the 7591 has more gain which would be useful for 2 stages. I like a 5K OPT myself for a few tubes including 300b and 2a3, but there you're talking some extra expense. Both are possibilities if you want to go there.
I've been using a very similar topology with EL33 into EL12n and that sounds pretty good. These are European tubes, though, and 7591 is more common stateside. When I replaced the EL12n with an EL34 in triode the sound wasn't so good, though it wasn't a premium pair and they were fairly used. And as stated the 7591 has more gain which would be useful for 2 stages. I like a 5K OPT myself for a few tubes including 300b and 2a3, but there you're talking some extra expense. Both are possibilities if you want to go there.
KT66 in triode mode are very linear ("poor man's 300B"). Should work pretty well with those 5k primary OPTs.
hifi, not guitar.
I've thought about doing a 300B or 2A3; the 2A3 was tempting because of cheaper tubes and, from what I remember when I was looking into it, the way the sound was described. I'd heard the 2A3 is pretty picky about speakers and the 300B is tricky to get silent. It would be more expensive but for a 'final' amp build it might be fun.
I'm much more of a follow the proven schematic builder than a designer/builder.
I've attached the schematic for the SE amp that is my main one. I also built the PP one but wasn't thrilled with the sound from it.
I've thought about doing a 300B or 2A3; the 2A3 was tempting because of cheaper tubes and, from what I remember when I was looking into it, the way the sound was described. I'd heard the 2A3 is pretty picky about speakers and the 300B is tricky to get silent. It would be more expensive but for a 'final' amp build it might be fun.
I'm much more of a follow the proven schematic builder than a designer/builder.
I've attached the schematic for the SE amp that is my main one. I also built the PP one but wasn't thrilled with the sound from it.
Well that's embarrassing. I missed that and the 5R4 thing. I have been feeling a bit "off" lately. Now I am worried about my eyes (or is it my brain?). 😱
what are the thoughts on these? (change the EL84 to 6V6)
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