Hi everyone!
I've got a pair of Edcor CXPP25 25W 7.6K PP tube output transformers and a variety of power transformers. I'm swimming in 6GV5, 6FJ7 and similar compactions and all sorts of oddball tubes, along with the regular common tubes.
It's been a while since I've built an amp from scratch. Any ideas?
I've got a pair of Edcor CXPP25 25W 7.6K PP tube output transformers and a variety of power transformers. I'm swimming in 6GV5, 6FJ7 and similar compactions and all sorts of oddball tubes, along with the regular common tubes.
It's been a while since I've built an amp from scratch. Any ideas?
Got a 300 VCT power trafo on hand? 400V of B+ will drive those OPTs to full power, and just about any horizontal sweep tube (and most of the vertical out pentodes) will handle the current. You dont even need the big ones, the little 18 watt compactrons are fine here. The GV5’s would work. The CT on the supply will give you 200-ish for the screens. You may need to drop that somewhat depending on the tube, but regulating 210 down to even 125 isn’t too much trouble. Looking at the GV5 curves, it looks like 125 would be enough, 150 for sure, even if the tubes were starting to get weak. Don‘t use a lot more Vg2 than you need, as excess just contributes to glowing grids and won’t give more power. With this plate/screen relationship in place, tube like this work fine in pentode mode with “conventional” front ends. Doesnt take any more to drive them than a pair of 6L6’s does.
I've got a Hammond 373BX, a 370HX and other assorted transformers. I had a bad case of Gear Acquisition Syndrome (GAS) about 15 yrs ago and I've probably got all the parts I need several times over.
I guess I'm wondering if I can take a popular vintage schematic and tweak it for what I have.
I guess I'm wondering if I can take a popular vintage schematic and tweak it for what I have.
I don’t see why not. I built a 135 watt bass amp out of four 26DQ5’s, and the rest of the circuit was nothing unusual, at least topologically (All off-voltage tubes doesnt count - a Mullard circuit is still a Mullard circuit). You just need a way of generating that 120-200 volt screen supply without exploding mosfets. From there the drive requirement is about the same as what you’re used to. And you could use TV tubes in your front end or the usual 12AX7’s and AU7’s. Two 40 mu lowish rp triode stages gives about the same overall gain as an AX7 and an AU7. You’ll be running a little further off the res, but hey it’s DIY.
I've got a plan forming.
I'm thinking of adapting a 5e3 circuit, because it is fairly simple.
The plan so far is to use the following, because I have them:
OPT: Edcor CXPP25
Output tubes: 6GV5 or maybe a 17AV5 (if I can figure out how to run the heaters)
Open decisions:
~ The 12AY7 has gotten too spicy $$$ so I need an alternate. I'm thinking either a 6J6 or 12AT7, which I've got plenty of. I'd expect some small tweaks are needed to accomodate the 6J6. Let me know if anyone has tips.
I'll need to determine the best OPT for the job too. I've got a hodgepodge of them around, or may have to sell some off to buy the "right one".
I've got most of the smaller bits and pieces for the job too.
I'm thinking of adapting a 5e3 circuit, because it is fairly simple.
The plan so far is to use the following, because I have them:
OPT: Edcor CXPP25
Output tubes: 6GV5 or maybe a 17AV5 (if I can figure out how to run the heaters)
Open decisions:
~ The 12AY7 has gotten too spicy $$$ so I need an alternate. I'm thinking either a 6J6 or 12AT7, which I've got plenty of. I'd expect some small tweaks are needed to accomodate the 6J6. Let me know if anyone has tips.
I'll need to determine the best OPT for the job too. I've got a hodgepodge of them around, or may have to sell some off to buy the "right one".
I've got most of the smaller bits and pieces for the job too.
I’d use the AT7’s if you have them. The problem with the 6J6 is the common cathode, which you may or may not want. Could use a mix, where the J6’s are used where you can stand grounded or otherwise common cathodes. I tend to stay away from those types and buy ones with separate cathodes so they can be used in more places. Plenty of $3 dual triodes out there if youre not dead set on 6SN7 (6CG7) linearity. Most of them run about 40 mu.
When optimizing your power transformer dollar, don’t be afraid to think outside the box. Putting two in series that you already have may be the right answer.
When optimizing your power transformer dollar, don’t be afraid to think outside the box. Putting two in series that you already have may be the right answer.
That's what I said too. But the 5E3's 12AY7 stage IS common-cathode, for two guitarists on one amp. And incidentally for interactive tone effects. Which means plate resistance matters. And neither 6J6 nor 12AT7 (both VHF tuner tubes) datasheets cite the low current used in this audio amplifier.
And "everybody" puts a 12AX7 in the front end of a 5E3.
But recent events limit supply of affordable tubes.
And 5E3 is not noted for linearity.
It's all in the fingers. Use the tubes you can get.
Good points about the 6J6. Indeed new Deluxe amps use 12AX7 competely as well.
FWIW I did a light modification to an AX84 Firefly where a 12AT7 drives a 6N6P output stage, and it's wonderful.
Oh I just recalled I have a weird Bogen 15 watt PA amp that may be a good iron doaner too.
FWIW I did a light modification to an AX84 Firefly where a 12AT7 drives a 6N6P output stage, and it's wonderful.
Oh I just recalled I have a weird Bogen 15 watt PA amp that may be a good iron doaner too.
Alright so a plan is coming together.
I've gotten a Hammond 290BBX for the job, but it's heater supply is insufficient for the hungry 6GV5 tubes. I've got some ancient dedicated heater transformer that has been wanting for a purpose. I'm a bit tempted to try it with a beefy choke in place of the first B+ supply resistor, because I have it. This is going to be a heavy 4 transformer amp!
6j6 for the input stage. I'll probably go with a 5U4GB rectifier, because I've got a pile of those too.
I'm also going to build the circuit to support a 6GB6 output if the 6GV5 doesn't work out.
I'm using a tubedepot PCB to keep things neat, and a Hammond 12x8x3 chassis that I'll have to paint. Either red or a dark hammered finish. Red might be louder though. 🙂
I've gotten a Hammond 290BBX for the job, but it's heater supply is insufficient for the hungry 6GV5 tubes. I've got some ancient dedicated heater transformer that has been wanting for a purpose. I'm a bit tempted to try it with a beefy choke in place of the first B+ supply resistor, because I have it. This is going to be a heavy 4 transformer amp!
6j6 for the input stage. I'll probably go with a 5U4GB rectifier, because I've got a pile of those too.
I'm also going to build the circuit to support a 6GB6 output if the 6GV5 doesn't work out.
I'm using a tubedepot PCB to keep things neat, and a Hammond 12x8x3 chassis that I'll have to paint. Either red or a dark hammered finish. Red might be louder though. 🙂
If you want optimized power to weight ratio, you almost always end up buying all new parts. Using what you have on hand, a heavy 4 transformer amp is often the end result.
You mean 6BG6, right? That’s the one that looks like a 6L6-oid with a top cap. I don’t consider 1.2 amp heaters especially hungry anymore since I started playing with cheap tubes. A lot of the cheap stuff takes 1.2A - even the 10 watters that can put out some juice at only 300V. The big boys take up to 2 and a half amps.
Fire engine red would make it louder, for sure.
You mean 6BG6, right? That’s the one that looks like a 6L6-oid with a top cap. I don’t consider 1.2 amp heaters especially hungry anymore since I started playing with cheap tubes. A lot of the cheap stuff takes 1.2A - even the 10 watters that can put out some juice at only 300V. The big boys take up to 2 and a half amps.
Fire engine red would make it louder, for sure.
Maybe a 6GB5? If so, they can make over 100 watts per pair when squeezed hard enough. I was looking to make a cheap 100 WPC HiFi amp board with 13GB5's when they were on the dollar menu some time ago. About the time I had the board ready one person (or corporate entity) bought up ALL the readily available stock of 13GB5's in the US, over 20,000 tubes. Stan (ESRC) wouldn't tell me who, but he said it was a US buyer who got about 9,000 of them from him. Without a large supply of cheap tubes my amp design became yet another dead project.
The 13GB5 amp did get reborn several years later with another even smaller $1 Compactron tube. This amp only makes 80 WPC on a regulated power supply or 50 WPC on an Antek toroid. These three tubes all have the same guts. The tube on the right is one that I experimented with a lot in my youth as they were commonly found in dead 1950's vintage TV sets. The center tube is the same thing in a Compactron bottle. The tube on the left is the same thing again with it's plate wings clipped a bit so it will fit in a tiny (smaller than a 6V6GT) Compactron bottle. It is seen in the pictures cranking out 80 WPC without anything glowing inside. The older octal version will easily do 100+ WPC. Stan had about 2000 of these little tubes when he passed, but nobody seems to know where all of his 4 million tubes went. Rumor has it that his Orlando warehouse is now empty, but I have not seen any real evidence of that, like say a picture through the front window.
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6BG6, 6GB5 - could have gone either way. But I ventured a guess it’s was the former, since it’s closer to what you would “normally” use for a guitar amp. $8 vs $5 at vacuumtubes.net, but that socket will cost you $3 more so it’s a wash there. The economy might have been different when tubes could be had for a buck, and when magnoval and novar sockets were as common as dirt. Neither is true anymore. Trying to make watts, the GB5 is better - more heater power = more mA of plate current for a given screen dissipation. Your pic shows a novar base, data sheet says magnoval. Which is it - really?
I prefer octal sockets because they are as common as dirt and easier to solder to, but I’ve since found a good source for compactrons. And plenty of 17-ish watt sweep tubes out there, many without top caps saving an additional $3 a bottle. The total cost is the tube, the socket, and the cap.
I prefer octal sockets because they are as common as dirt and easier to solder to, but I’ve since found a good source for compactrons. And plenty of 17-ish watt sweep tubes out there, many without top caps saving an additional $3 a bottle. The total cost is the tube, the socket, and the cap.
Hi George!
Yeah I meant 6BG6. I've got some Phillips ECG 6BG6GA and socket adapters, which are Super 6L6's with a different pinout and top cap. However I got way more 6GV5's from ESRC's $1 tube blowout sale, so I'd rather use those. I wouldn't expect any surprises using those instead of 6V6's. But I've also blown up a lot of stuff making those assumptions.
Yeah I meant 6BG6. I've got some Phillips ECG 6BG6GA and socket adapters, which are Super 6L6's with a different pinout and top cap. However I got way more 6GV5's from ESRC's $1 tube blowout sale, so I'd rather use those. I wouldn't expect any surprises using those instead of 6V6's. But I've also blown up a lot of stuff making those assumptions.