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Old 18th August 2006, 05:53 PM   #1
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Default Dyna ST70 experience bypassing input tubes

I have a Triode Elec input board in my ST70 and always had to much sensitivity and tons of hum. I know its the PT vibrating causing the hum but I don't want to spend more on a tranny at this time. I rewired the EF86 inputs to triode to drop the gain and didn't notice much of a difference. Using a SRPP 12SN7 preamp it's still way to sensitive for the pre's volume pot to really be usable (10 o'clock is very loud). So the other day for fun I decide to bypass the st70 input tubes altogether and go straight to the phase splitter (which as zero gain). Now the volume is around 3 o'clock for decent output, BUT there is NO hum! So my idea now is to scrap the input board completely and handwire in a longtailed phase inverter instead, fed straight from the preamp, not an addition gain stage. Am I on a realistic road here? I know the longtail PI has some gain but I have no clue how much. I'm hoping a 12AU7 (or maybe 12AT7) might provide a little more gain after my pre, but still not so much that I get so much hum from the amp's power tranny. What I don't know is how much gain a longtail PI has? I've googled a lot but can't find any info, and the few books I have don't discuss the gain of this PI either.

Thanks for any help!

Regarding the hum, I have done all the usually checks to confirm it's not grounding, etc.
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Old 18th August 2006, 11:48 PM   #2
SY is offline SY  United States
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Broken record time: all the formulas and design info you need is in "Valve Amplifiers" by Morgan Jones.

A long tail pair will give you half the gain of the same tube set up single ended. A 12AT7 is a very good choice- you'll probably end up with a gain of about 20, which is just right to tickle EL34s with a volt or two from the preamp. And my measurements have shown that tube to have excellent linearity at 1.8-2mA and 150V on the plate.
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Old 19th August 2006, 01:07 AM   #3
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Thanks SY. You know, I had that book but now I can't find it anywhere. I'll have to spring for another copy. I have MJ's more recent "Building Valve Amplifiers" book and it's good as well for the layout, etc.

Thanks again.
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Old 19th August 2006, 04:44 AM   #4
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The 12AT7 has enough gain to drive EL34s with a CDP direct in, IF loop NFB is not used. The Hafler designed O/P trafos in the ST70 were always intended for use with loop NFB. So a preamp with a little gain is indicated. The 'SN7 SRPP line stage has more than a little gain, as the "stock" ST70's woes show. IMO, a tube with a smaller mu than the 'T7 is appropriate, in this instance. The 6J6 and 12AV7 are candidates. The less linear 6J6 may be more synergistic in combination with PP "finals".

In any event, consider building an external B- supply and adding a constant current sink (CCS) to the differential splitter's "tail".

SY has a suitable CCS. Another CCS option can be found in the "El Cheapo" project here.
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Old 19th August 2006, 08:09 AM   #5
kegger is offline kegger  United States
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Actually in the st-70 you have a 55v tap, if you use a voltage doubler
with proper caps and whatnot maybe even a spare 70 choke you'll end
up with about 130v which you could use to make a neg tail voltage.

Model it up in PSUDII, I think you'll like what you see.
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Old 19th August 2006, 12:35 PM   #6
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You don't need that much tail voltage. It only has to be enough to allow for the CCS minimum plus one half the signal. The existing bias supply will work perfectly if it's filtered a bit better.

Eli is correct, I was assuming no global NFB in my 12AT7 long tail suggestion.
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Old 19th August 2006, 01:45 PM   #7
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I wonder if it's really safe to draw approx. 12 mA. from the ST70's bias supply. As Kegger indicated, the ST70 bias supply is derived from a tap on the B+ winding. I'm concerned that the significant draw will cause problems in the B+ supply. Fisher and Scott used separate windings when the bias supply also powered phono stage heaters. FWIW, I favor a separate B- supply on the better safe than sorry principle.

Mouser carries a $12 24 VDC "wall wart" that should do the job. Look here. Wrap the WW cable around a ferrite ring several times to keep noise out of the amp.
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Old 19th August 2006, 02:02 PM   #8
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A separate supply couldn't hurt, especially because you can use something better than a half-wave rectifier. But taking another 12mA from the transformer is really not a problem- I did this with my modification of the Curcio ST-70 for exactly this purpose.
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Old 19th August 2006, 03:06 PM   #9
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A long tailed pair or diff pair is a really useful front end - I use this with a Morgan Jones CCS under it in amps. I found like you did that it makes sense to plan on a three stage setup. If the first stage is the preamp and the gain is sufficient, then if it's only for your own use you can have an amp of less than usual sensitivity and just use two stages in it - probably without feedback (if you do this you may like to convert the ST70 to use 6B4Gs). If you are in UL, Menno van der Veen has some interesting stuff on his psuedo triode setup for UL, which has a larger screen resistor and some added componants around the grid of the EL34.

If you have a diff pair input to your amp, you also have the possibility to go balanced. This allows a simple shunt stepped attenuator (a 12 step one is simplicity to make) in the preamp or indeed the amp. Personally I've gone over to DHTs in my preamp, and I'm over the moon with the results - another design choice. You could even use a DHT for the diff pair, like 19, 1J6G, 3B7
DCC90 or 3A5 - all double triodes with different basings. If you do that you'll need a really good DC filament supply. There should be some ideas on this forum if you do a search - something like a RonanReg or a Tentlabs one. The filament supply should be current based - sounds better than voltage regs (with or without a common mode choke) but a 2 stage DC supply with voltage regs (e.g. LM1084 type) will get you up and running. Andy
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Old 19th August 2006, 05:03 PM   #10
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Quote:
A separate supply couldn't hurt, especially because you can use something better than a half-wave rectifier.

This thread's originator, "tubesmuggler" is Canadian and (IIRC) Allied distributes directly in Canada. So, let's look for parts in Allied's catalog.

Stock # 967-7423 is a Triad F3-48 48 VCT/50 mA. trafo. Stock # 950-1080 is a MBR20100CT common cathode twin Schotky diode rated 100 PIV/10 A.

Start with a "normal" CT full wave setup, but ground the common cathode and take the raw B- from the trafo's CT. Use a CRC filter assembled from 100 WVDC parts. Keep the 1st filter cap. on the small side, say 15 muF., as the conduction angle will be relatively large and ripple overtone levels low. Follow with a 1 KOhm inductive WW resistor and a big 2nd 'lytic.
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