ST-70 / AA-100 / was 7591 now 6BG6G

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ST-70 / AA-100 / was 7591 now 6BG6G

Postby TomMcNally » Fri Mar 02, 2007 9:30 pm

So about a year ago, I built an ST-70 clone with Shannon's driver board, Heathkit AA-100 iron, and 7591's ... the reason I used 7591's is that I bought them to restore the Heathkit, and then realized it was a piece of crap.

I used a nice chassis from Yaeger Audio. The amp sounded pretty decent, but the tube sockets, new ceramic/gold, were junk and the tubes went on and off. They were too cheap to tighten. Yesterday, I yanked all 4 of them out, and put in the "Johnson" style ceramic sockets under the chassis. I also converted the bias test points, which were banana jacks, to plate jacks, an put pin jacks in for the bias test points, and wired it to use 6BG6G's.

Image

As I should have expected, my dozen eBay tubes were all different brands and half bad, so I have some Sovteks on the way from Ned. So far, it sounds pretty good, I didn't look at the bias or check any voltages, I'll do that Monday when the tubes get here.

Image

It looks cool anyway. The plate wires are bright orange Teflon, they look kind of yellow in the pics.
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Postby mesherm » Fri Mar 02, 2007 9:46 pm

I had to buy a bunch of NOS 6BG6Gs before I got four matching RCA ST shaped ones so I have 6BG6G odds and ends too...lol
I also have a matched set of 4 Raytheons and a matched set of 4 Philips/ECG but they are not the nice ST shape so they stayed in the tube box.
I think you will like the sound of the 6BG6s..they always sounded a bit snappier to me (if that makes any sense). Nice build Tom.
Mike
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Postby TomMcNally » Mon Mar 05, 2007 4:38 pm

Wow - I got my Sovtek 6BG6G's today from Ned's http://www.tubezone.us new for only $ 5.95 each ... amazing sound. Without any mods, I can only get the bias to set at about 60 ma per tube. They aren't running red ... do you think I should try to lower that ?

... tom
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Postby mesherm » Mon Mar 05, 2007 5:07 pm

At $6 a tube I say keep them at 60ma if they sound good. I was running my NOS tubes at 47-50ma which is about the lowest I could get without boosting my raw bias supply to -100v. There was the slightest hint of dull red in some spots on the plate but the tubes seemed to be happy. They must be pretty rugged.
I have a suspicion their response is not quite linear and the plate current increases more as the grid voltage approches 0 giving transients a quicker rise time but thats just my gut feeling. I like the sound though.
Maybe I need to drop in at Uncle Ned's and get me a set too...
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Postby TomMcNally » Mon Mar 05, 2007 5:22 pm

The box of junkers I wasted $ 10 on on eBay were a mess. A couple with open filaments, one must have had a short and took out a cathode resistor (I noticed that today when one had 38 volts on the test jack) and another one (while still playing) go sooooo hot I smelled it - it was bright red. So they are going in the trash ... not worth messing with. I think I've learned not to buy any untested tubes - even from dealers, because they are usually bad. I wasted a penny (and $ 5.95) shipping on a pair of 832's from eBay and one had cracked glass that looks like it happened in 1956. haha

Thanks for the advice Mike - I'm gonna leave this amp alone - it's sounding good. By the way, Ned has the right ceramic plate caps for the 6BG6's for a mere $ 1.69 each, and they look, fit and work great. Fresh stock too, the silver isn't tarnished.
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Postby mesherm » Mon Mar 05, 2007 5:42 pm

I had a set of GEs that were 6BG6GAs and not the older ST shape that I tried when I first built the amp but even with my bias at the highest starting level as soon as the heaters got warm a couple tubes' cathode currents started going off my meter scale and before I could get some kind of a balance, one went red and I started to smell that bad smell of roasting componenets. I shut the thing off and almost canned the whole project until I got the guts to try some other tubes. I still have those GE tubes that I suspect were specifically made for TV sweep duty only with like -150 volts on the grids. They don't work for tube amps. The ECG Philips set and the Raytheon set both work fine as do most of the older ST G versions. Sounds like those Russky tubes work ok too...what bias voltage do you have on them to get 60ma?
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Postby TomMcNally » Mon Mar 05, 2007 7:42 pm

I'll check the voltage and let you know - it sounds so good I don't wanna flip it over ! The power transformer (Heathkit AA-100) is running pretty hot. It shouldn't be - the 6BG6G filaments are 900 ma each, the 7591's are 800, and the load should be less than the AA-100, it's less tubes.
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Postby mesherm » Mon Mar 05, 2007 8:10 pm

No great need to know on my part...was just curious. Don't flip it over just for that.
I know old ST70 060 potted PTs always ran hot. I had an ST70 I sold that had an original 060 PT that always got too hot to hold after about 90 minutes but it never quit. Is the Heathkit PT potted also?
Sounds like the heater current is more like 1.2 amps to me...seems I remember reading that somewhere when I was researching 6BG6 types but I don't recall now.
Your report on those tubes has prompted me to get a quad from Ned also. I will re-convert my 6BG6 to EL34 amp back to 6BG6 pinouts and see how it sounds. I was unimpressed with 6CA7EHs. I needed to get an octet of EL84 types anyway for new project and Ned has a great price on those too.
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Postby TomMcNally » Mon Mar 05, 2007 8:56 pm

The transformer isn't potted, it's above the chassis style, and the outputs are through the chassis. I pointed my trusty Whisper Fan at it, and it cooled down. The GE and Sylvania data sheets both say 900 ma, maybe the Sovteks are heftier ?

I looked at the AA-100 schematic, it's a 50 volt bias tap, and they only ran 14 volts fixed, no adjustment at all originally. There is one series resistor I can mess with and see if I can get the current down a little, the tubes and transformer will run a little cooler.

My ST-70 has that tar-potted power transformer, it does run hot, and looks crappy.

... tom
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Postby TomMcNally » Thu Mar 05, 2009 7:36 pm

After reading some new posts regarding bias, I revisited this
amp for the first time in a long time. I tweaked the value
of the ground leg resistor in the voltage divider in the bias
supply, and got the tubes within range, I set them to
45 ma for now. It sounds great with a set of Russian
6BG6G's I got from Uncle Ned a while ago. I had robbed
the Mullard GZ-34 for another amp after a JJ blew up,
but I found an Amperex in the junkbox and it's good.

I'll jack the current up to 55 ma and balance the bias with the
pot on the ST-70 board later ... but for now the plates aren't
bright red and the amp is kicking !

... tom
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