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Dynaco ST-35 Circuits

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Hi Folks,

Right now I'm thinking of building a Dynaco ST-35 clone as my first tube amp this summer. I have an ok understanding of electrical basics, and have done a few diy electronics projects before, but this is my first step into the world of Valves. I chose the St-35 because of its excellent reputation, relative simplicity and availablity of decent new production tubes and well regarded transformers. However I want to use the glass and iron to it's fullest potential, I'm not married to the original circuit. Basically the ST-35 seems hit my performance and output power vs cost and size goals perfectly.

I want to actually understand the circuit before I start building, and that is coming along well. I'm learning a lot about tube amps. I feel like I've got the power supply down pat, and after looking at the original, and few more modern designs such as the DIYTube stereo-35, and doing some reading I think I've come up with my own design that I'm pretty happy with on paper.

Anyway, now on to my real questions. One thing I've found very useful is looking at different circuits and modifications of the ST-35, then understanding what the differences are and why. I was hoping someone with more tube knowledge than the meager amount I possess could look over the ST-35 circuit and make some recommendations on what improvements are possible, if any.

The schematic is available here: http://www.tnt-audio.com/ampli/dynaco_st35_manual.pdf
Schematic is on page 12, values are at the bottom right of the page.

For the '35 the go-to modification is the addition of the "enhanced fixed bias" circuit, beyond that not a lot exists for the actual amplifier circuits. One popular thing to do is to change the 7247s to a 12AX7 and 12AU7 with each channel using half, this seems to be more for cost and availability of tubes. From there some people change out the AX7 for a 5751, or the AU7 for a BN7 if they have enough overhead in their heater circuit for the increased filament current. I've already read up and looked over these mods, so don't need them discussed here.

The DIYAudio circuit doesn't really depart from the original much in the amp circuit, the only value differences of concern are in the NFB circuits where C6 in the above schematic is dropped from 18pf to 15pf, and R10 is dropped from 27k to 20k3 (my understanding is this is to facilitate the use of the modern OPTs with 8 and 4 ohm taps, instead of the 16 and 8 ohm taps of the original).

One suggestions I've read a few times is "more NFB", but I'm at a loss for what exactly "more" means with regards to NFB, I'm also hung up on how to simulate the NFB circuit as so far my results don't really make sense (I had another thread about this).

The only other mod I've seen mentioned was increasing C4 and C5 from .22uf to .47uf. Unless I'm mistaken these are providing coupling for the power tubes, for DC blocking. They have a -3db point of 1.5hz, increasing their value drops that to ~.7hz. Unless there is another function/effect I am missing I don't see why one would do this.

As for the power supply I have read that the power transformer runs pretty hot on these. One cause of this is the heater circuit, and I've got a 6v 6amp transformer here collecting dust, so I'll likely use a separate xformer for the heaters. Additionally the stock PT is rated at 120v/330v, 0v, 330v 180ma, but looking at tube datasheets, etc., it looks like the amp will run somewhat close to that current rating. I can get a Hammond 290EX (120v/330V, 0V, 330V 276ma) for basically the same cost after shipping, duty, etc as the Dynaclone PT. The Hammond's heater windings aren't up to snuff for the ST-35, but as I said I won't be using them anyway, so is there any reason I shouldn't go for the overkill Hammond? I figure I might get better regulation and less sag with the bigger PT, plus running cooler would extend it's life and that of near by components. The main plus would be a more useful transformer for future projects should I ever rob the '35 for parts.

Finally, the '35 has about 500k input impedance. I'll be using a preamp, not modifying the '35 to be an integrated. Beyond R1, R2, C1 and C2 would I need to make any other changes to the amp if I changed the circuit to have an input impedance of 50K or 100k? I don't see this having any impact on the circuit after the first tube, so no other changes would be required, but as I said I'm new to tubes. (the preamp requires the device it's connected with to have an input impedance of 100K or less).

Thanks in advance and sorry for the long post!
Increasing the size of coupling capacitors has a downside: when the grid is overdriven it forward conducts, like the diode that it is, and charges up the coupling cap. The larger the cap, the longer it takes to bleed down, back to normal bias. A good compromise is about 4Hz, or about 1/4 second hang time. Many, many amplifiers ignore this, and are the poorer for it.

One possible issue with the original ST35 (and SCA35) is their very high B+ voltages, common in those days. Today we'd probably make a more conservative design with maybe 300 to 325 volts B+.

The original circuit is very elegant and clever, but you may or may not want to include the positive feedback. It's not much, and kind of a tossup anyway.

If you're considering separate first and second driver stage valves, and have a spare heater winding, you might want to use it for the second stage's heater, and raise it up about 100 volts from ground, to minimize the heater-to-cathode voltage strain.

All good fortune,

I'm don't remember coming across that Dynaco site, I'll defiantly have a look at it.

Hi Chris,

Thanks for your reply. So you would recommend something more like 80nF for those coupling caps. What is the reason for prefering lower B+ in modern amp designs, is it tube life or performance?

If I'm understanding correctly R4 and C3 are providing positive feedback? I've read a couple anecdotal accounts of people removing the positive feedback and preferring the sound with it in place, definitely something I could experiment with once the amp is built though.

If I build the ST-35 circuit I will definitely use separate first and second driver stage tubes, So I will keep your suggestion for the heaters in mind.


I had mistakenly written off the Baby Huey thinking it was only 5-watt output. Aside from the more simple circuit and use of a single 12AX7 per channel, what do you feel makes it superior to the ST-35? Have you listened to both?

(In my first thread on tube amps I had been considering several 5-watt designs, but have since decided they might be a little underpowered for my purposes)
However I want to use the glass and iron to it's fullest potential, I'm not married to the original circuit.

Then build an "El Cheapo", with UL/triode mode setting switches! Other good designs are "Baby Huey" and "Red Light District".

The Z565 O/P trafo used by Dyna in the ST-35 is a price/performance champion. To date, the Z565 is the "Gold Standard" for "El Cheapo" builders. Contact member Jeff Yourison to get the benefit of his considerable experimental experience with the design. Look here to see what Jeff ended up with. Gorgeous!

The in production 6V6 can be used with zero parts value changes. To use EL84/6BQ5 O/P tubes, changes are needed only in the RC cathode bias networks.


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I had mistakenly written off the Baby Huey thinking it was only 5-watt output.

If wired in triode. If wired in UL it will have the same power as the ST35.

Aside from the more simple circuit and use of a single 12AX7 per channel, what do you feel makes it superior to the ST-35? Have you listened to both?

I have heard ST35 stock, and we buiit an El Cheapo variation that ate it for breakfast (with EL84)

Baby Huey is topologically similar to El Cheapo, with the CCSs in each of these they are more complex than ST35.

Maybe Dave Gillespie's fixed bias EFB mod would even things up, as it's said to help the sonics, besides output tube life. Another bonus is 2-3 watts more power.

Here's an ambitious ST-35 clone with thje EFB mod.

diytube.com • View topic - I did it!

Of course that EFB mod could be used on EL Cheapo also if one wants to try the LTP circuit.

More info here.

Dave's Lab

It would help compensate for the higher line voltages we have today, by allowing you to lower the bias to get the dissapation down. Better to be able to run any new production EL84/6BQ5 in an amp also.
The power supply in the stock ST-35 is pretty terrible too in comparison. Playing around with PSUD it was showing about 5.5V ripple on B+, under similar conditions the El Cheapo gives about 7mV, and has a whole heck of a lot more energy stored up in it. The CLCRC power supply I drew up has pretty similar performance to the El Cheapo's.

I'm enjoying the discussion and information that everyone is providing.

Dave, can you elaborate on the CSS, and how much that will influence the amps performance? Though I can see the differences I don't know enough yet to even guess at the magnitude of their real world effects.

Two questions coming out of the El Cheapo Circuit,

1. The diodes on B2+ are supposed to isolate them from B+? Is the intention that a transient spike in B+ current wouldn't be able to draw from the B2+ reservoir capacitors? Does this actually have much of an effect?

2. The EL84 seems to be the go to in this power and price range, is there any reason to consider a 6v6 el cheapo over an El84 one? (besides that they look cooler!)

I wish I had the opportunity to listen to either the Baby Huey or El Cheapo. Thanks to everyone for trying to help me sort out the advantages of these two amp circuits over that of the ST35 with an improved power supply, and EFB. Like I said I am in no way set on the '35 circuit, though I will likely use the Dynaclone OPTs due to their excellent reputation and price point. Is there any chance someone could describe some of the tangible benefits of the Baby Huey or El Cheapo over the Dyna when using the same OPTs?
The diodes on B2+ are supposed to isolate them from B+?

You've got the idea. The diodes operate as check valves. Approx. 15 cents/diode is cheap insurance against instability. The driver and O/P stages are well decoupled.

The EL84 seems to be the go to in this power and price range, is there any reason to consider a 6v6 el cheapo over an El84 one?

Yes! The 6V6 makes a better triode than the EL84. Build with mode selection switching capability: UL for "cujones" and triode for finesse. Triode mode RULES, when listening to small ensembles, like string quartets and jazz combos.
Dave, Unfortunately I moved from Victoria to Halifax in August.

Eli: What is the output power on the El Cheapo?
Dave stated 3.2w in triode mode with EL84s, what would it be in UL with 6v6?

I'll be driving low 90dbl sensitivity speakers in a small room. Switchable modes interests me a bit, as I listen to a wide variety of music, everything from solo cello, Jazz (early Miles Davis and similar era), delta blues, through to metal and garage rock. I do mostly quiet listening levels as not to disturb my better half, but occasionally like to blast it out.
The "Victoria Boys" set up with strict Class "A" O/P stage operation. Usually, an "El Cheapo" is set up with "shallow" Class "A" O/P stage operation and yields about 6 WPC in triode mode and 12 WPC in UL. "Shallow" Class "A" means that the tubes alternately cut off, when peak O/P power is being produced, but most of the time both "finals" are conducting.

A power O/P of 12 WPC will be LOUD, in combination with low 90s sensitive speakers and a small room.

Eli do you have a schematic of the amp in UL mode?

No schematic, but a flip/flop from triode to UL is very easy to do. :) Just move the resistors that tie each g2 to a plate from that plate to the corresponding UL tap on the O/P trafo. A DPDT switch in each channel gets the job done.

Do not move the mode setting switches, while the unit is powered on!!

Quad power tubes are definitely in consideration for a future project, if I end up liking tubes for my personal amp, I'll be building something more powerful for our living room system. Right now desk real estate is a little limited which brings me to my next question for Eli.

The dogs-breakfast of transformers (meant in an entirely non-offensive way) in the el cheapo will certainly take up some chassis room. Instead of the voltage doubler, etc with two 120v,120v transformers, could I use an appropriately rated center tapped transformer with a bridge? + side of the bridge for B+, - side for B-, both grounded to the center tap. A quick look in PSUD tells me the resistor in the B- supply would have to be increased. It might be easier to picture as a full wave rectifier for each, one positive, one negative. Looks like 200v, 0, 200v would work with the B- R increased to 12k. That gives practically the same B+ and B-. Keeping with the el cheapo name, it looks like that would also be cheaper than the specified transformers.

Also the use of the 12AL5 is to provide a delay to B-?
hmm, turns out I misread the PT datasheet, and was off by a large factor for current rating, ignore the above.

Looking at the input coupling values on the El Cheapo wouldn't it roll off the bass using OPTs like the ones discussed here. I'm getting -3db points of around 25hz, when everything I've read has suggested that 2-5hz is the way to go.
Years ago, when the "El Cheapo" project started, we were looking for a way to get quite reasonable performance and hold the cost of parts to approx. $200. Increases in materials prices have made the $200 goal no longer feasible, but the low cost/high performance "mantra" remains intact. Yes, a custom power transformer would be "better", but kiss the low cost good bye. :crying: Given that 2X N68-X isolation trafos currently cost less than a single N-77U, that's the current recommendation. :D While "a dogs breakfast" may aptly describe the assortment of off the shelf power magnetics employed, the group provides enormous "bang for the buck".

Yes, the Greinacher, AKA "full wave", doubler B+ supply is very cost effective. That's far from the complete story. Properly executed, the Geinacher setup more than holds its own against other arrangements. "Idiots", such as Fisher, Hegeman, Marantz, and McIntosh, used the setup in some of the most revered equipment ever manufactured! I was more than happy to follow in their footsteps, while being frugal. :yes: ZERO technical corners were cut to contain expense in the PSUs of "El Cheapo". It's not possible to get a good amp from a crappy PSU! For instance, the 12AL5 hybrid bridge negative rail is inexpensive, noise free, and slow starting. If the grid in a 'T7 section becomes positive with respect to the cathode, failure is highly likely. :mad: BTW, approx. 20 years is the service life of an 'AL5, in this role.

Frugality is good. Parsimony is bad.
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