Dynaco ST70

[cwteoh]

Hi, if you are build your own Dynaco, I wish I can get some info from you, because I am thinking of build 1 for my own. I need some experience expert for guides.

I got the original Dynaco ST70 schematic with me (shown below). With the following tube I could able to start my construction.

4 x E34L quad matched
2 x 7199
1 x GZ34

[SY]

7199s were difficult to source last time I checked. Have you located some? What transformers are you planning to use?

[Frank Berry]

Replace the 7199 tubes with 6U8's. Performance will be the same. Be aware that the pinout is slightly different.

[cwteoh]

what do you think about the following schematic. It has been modified by one of my local DIY guy. Please advise, thanks

[Eli Duttman]

In the 2nd circuit, I'm somewhat dubious about a 12AU7, with it's mu of 20 having enough gain, especially since there is degeneration in the O/P stage due to the unbypassed cathode resistor.

In general, I LIKE the 2nd circuit. I would quibble with the details. The reality of tubes not being TIGHTLY matched can't be ignored. "Fixed" bias with individual adj. pots. for each tube allows exact setting of idle currents, which compensates for mediocre matching. I would use a tube with more gain than the 12AU7, say the 12AT7 and instead of grounding the non-inverting triode's grid, use it as the point of application of negative feedback picked up from the O/P transformer's secondary.

[Jax]

Something like this?



Still subject to tweaks but a prototype is up and running just fine.

[Eli Duttman]

Jax,

That's impressive! There's very little I would change. Use the less expensive, but EQUIVALENT Locktal 7N7 instead of the 6SN7. Change the EL34 cathode resistors to 100 Ohms, for a "dose" of Dennis Boyle's combination bias.

FWIW, I had a 2 stage circuit with a 12AT7 as the phase splitter in mind using a less sophisticated CCS (an enhancement MOSFET).

[planet10]

Originally posted by Frank Berry
7199 tubes with 6U8's.

I've a whack of used 7199s -- even a couple cryoed new ones i'll never use... even more 6U8s.

This is the direction i'm going with my ST70 (compliments of Allen Wright):



I may well use 6H30Pi or ECC99 (althou with 6H23Pis on hand...)

dave

[Jax]

Thanks, Eli.

One of my goals in the design was to reduce the number of low frequency poles within the loop to a reasonable minimum. There are only two left, the coupling caps and the transformer. Differential stages is a good way to avoid decoupling caps combined with the DC-coupling.

I have been thinking of stabilizing the output stage bias with higher cathode resistors but 100 ohms? Should they be decoupled? You made me curious now

[SY]

p10- all you need is a Jensen JT11-P1 on the input...