Dyna ST70 to Dual-differential EL34 triode amp project - diyAudio
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Old 9th April 2012, 07:38 PM   #1
rongon is offline rongon  United States
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Default Dyna ST70 to Dual-differential EL34 triode amp project

I have an old beater ST70 that I keep experimenting with. I've stripped it down, once again (output tube sockets wore out). So I have a new project.

In honor of the late Arthur Loesch, I'm going to try to make this a "dual-diff" push-pull triode amp, sort of like the PP-6B4G amp from the "No Stone Left Unturned" article in an early issue of Sound Practices. It's also a lot like the 6L6 AB2 amp in a thread on this forum. But I'll be happy with 10 weenie little class A watts per side.

I've attached the proposed schematic below. It's very simple because I need to pack a lot of tube sockets in this very small ST70 chassis. (I'll be making a plate to go where the original PCB went. Point-to-point wired...)

In order to keep the current draw within reasonable limits, I plan on using 6L6GC's in triode instead of EL34's for the finals. That should save a lot in heater current. I figure I'll need it, because each 5687 will draw as much heater current as a 6L6.

I have solid-state rectification in the chassis already. So I won't be using the 5VAC winding. I hope that saves more stress on the old Dyna power transformer.

My major concern is exceeding the heater-to-cathode voltage limits of the 5687's at turn-on. The 5687 grids are DC-coupled from the 6DJ8 plates. I put in a pair of diodes from the 5687 cathodes to grids, so that the cathodes will be kept from going way up to the B+ while the tubes are warming up. Did I do that correctly?

I also floated the heater windings on +45VDC. Maybe that's not necessary. The original has the heater windings blocked from ground by a 20nF 1kV ceramic capacitor. Maybe that's all that's necessary? That part is still wired up.

Thanks for any advice on this (I'm afraid I'm just good enough at this to be really dangerous!).

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Old 9th April 2012, 10:29 PM   #2
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I won't argue with your calculations of heater current, but remember besides the 4 each 6CA7 and the 2 each 7199, the ST70 power transformer had heater current for four more 12AX7's. That is two PAS1 preamps worth.
Be sure to terminate the unused 5 VAC heater winding with 1k ohm 1 watt resistor to prevent excessive voltage from building up.
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Old 9th April 2012, 11:11 PM   #3
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The turn-on surge is going to be over 500 (over 525 in mine with solid-state diodes).

You're going to have to either limit that surge or increase the voltage tolerance of the filter caps.
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Old 10th April 2012, 12:26 AM   #4
rongon is offline rongon  United States
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Quote:
Originally Posted by indianajo View Post
Be sure to terminate the unused 5 VAC heater winding with 1k ohm 1 watt resistor to prevent excessive voltage from building up.
Hey thanks, I hadn't thought of that. How would I terminate those windings? You mean put a 1k 1W resistor from each yellow lead to ground?

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Old 10th April 2012, 07:37 PM   #5
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Quote:
Quote:
Originally Posted by indianajo

Be sure to terminate the unused 5 VAC heater winding with 1k ohm 1 watt resistor to prevent excessive voltage from building up.
I think I'll use that 5VAC winding to make a simple 6VDC supply for the driver tubes' heaters. Why not? A 5A Schottky diode, a 10,000uF 16V 'lytic and it'll be good enough, I'm sure. Not optimal, but oh well...

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Old 10th April 2012, 09:23 PM   #6
rongon is offline rongon  United States
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Does anyone know if the resistor value matters for triode wiring an EL34 or 6L6? (Resistor placed between screen grid and plate/anode.)

I don't have four of anything less than 1k ohms in 1W (metal oxide). I see that Eli D uses 1k ohms (carbon comp) there in the El Cheapo (6AQ5). But you usually see 100R or 220R, 1W.

I think the resistor is only there to inhibit the screen grid from drawing current. So I'd think 1k would work fine there. I'd also think a 1/2 watt resistor would be fine there, but when I've used them in that location, they always look like they're getting heated up too much. Never had one fail, though.

Am I missing something?

(Sorry, I searched but wasn't able to find a discussion on this particular subject...)

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Old 10th April 2012, 10:19 PM   #7
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Default Well, I don't know the answer but......

FWIW, Poindexter uses either 332 ohm or 150 ohm R (and a diode in series with the R) in the triode wired EL34 MM depending on which schemo you find.

I think you may struggle getting 6.3VDC out of a 5V winding.......it may be tough even with a 6.3V winding to get clean DC.

Great thread, BTW

Have you thought about throwing some powerdrive MOSFETs in there?
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Old 10th April 2012, 11:47 PM   #8
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Cool. I would like to watch and learn
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Old 11th April 2012, 12:24 AM   #9
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Originally Posted by boywonder View Post
FWIW, Poindexter uses either 332 ohm or 150 ohm R (and a diode in series with the R) in the triode wired EL34 MM depending on which schemo you find.
Yeah, that's what I expected. I've seen 100R, 150R, 220R, now 330R in the MM. Maybe I remember 470R in some amp somewhere. But the only one I've seen with 1k between plate and screen is Eli's El Cheapo with 12AQ5's.


Quote:
I think you may struggle getting 6.3VDC out of a 5V winding.......it may be tough even with a 6.3V winding to get clean DC.
I figure if I get 6VDC, I'll be OK. That would be only 5% low, which would make the tubes run cooler. There was evidence (from a Norman Crowhurst article?) that slightly undervoltaging the heaters makes indirectly-heated tubes last longer... Unless that has been refuted somewhere.

If I don't get 6V, I'll rip it out and feed the 6DJ8's and 5687's from the 6.3VCT windings.


Quote:
Have you thought about throwing some powerdrive MOSFETs in there?
Yes, but there's very little space in a Dyna ST70. Maybe if I used a 12AT7 as a LTP phase splitter with powerdrive MOSFETs to the EL34 grids... But I'm not sure I'd get enough voltage swing in, even from a 430V B+. I figure a 12AT7 as LTP phase splitter (unbalanced input, balanced P-P output) will only yield a gain of about 30, and I'd expect the 12AT7's Vgk to only be about 1.5V. So figure only a little more than 35Vpk output, which wouldn't leave enough room for NFB. I figure there's no way the stock ST70 OPT's are going to be long-term satisfying without a little NFB. I figure I'll need 5 or 6dB NFB.

I've built this dual-LTP driver before. It really sounds great when it works. It does tend to oscillate if not laid out well.

I spent a couple hours planning the layout for the dual-LTP driver. It's going to be tight, but I think I can make it work.

I have piles of 1980s vintage Philips ECG 5687WB and 6922 tubes around. I figure I should use them. I figure I'll get a gain of about 14 from the first stage (unbal in/bal out), and about 12 from the second stage (bal in/bal out). That'll give me gobs of voltage swing to eat up with NFB. Maybe Schade?

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Last edited by rongon; 11th April 2012 at 12:28 AM.
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Old 11th April 2012, 07:14 PM   #10
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Quote:
Originally Posted by rongon View Post

I figure if I get 6VDC, I'll be OK. That would be only 5% low, which would make the tubes run cooler. There was evidence (from a Norman Crowhurst article?) that slightly undervoltaging the heaters makes indirectly-heated tubes last longer... Unless that has been refuted somewhere.

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The dynakit PAS2 preamp runs the 12AX7's at 12 VDC. I've been running it since 1982 with two silicon diodes instead of the selenium, drops another volt off. Two of my 12AX7 in the PAS2 are 51 years old, and I replaced the other two in 1974 for popping, probably caused by leftover flux on the PCB and humidity, not the tubes.
The 5AR4 is a filament current hog, getting 6VDC out for a measly signal tube should be easy. Use schottky diodes.
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Last edited by indianajo; 11th April 2012 at 07:17 PM.
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