• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hello...and another ST-70 project.

8vgumby

Member
2010-10-24 10:02 pm
I figured I would finally post something, I've been lurking and reading on this website for quite some time now.

I am looking for help and advice where I can find it. I've been playing with audio stuff since I was about 10..11 years old. From that, I've expanded to a full HT arrangement, and have slowly begun to work on a true hi-fi setup.

This has progressively changed from a Carver MXR receiver and Acoustic Research Towers to my current NAD 3155, and Paradigm Monitor 7's.

My knowledge of DIY in this area has been limited to the random PCB projects in highschool/college, as well as general repair work on solid-state stuff (replacing caps, resistors, connections...etc..)

Recently unemployed, and returning to school, I find myself with an absurd amount of time on my hands, and a need to rekindle an old hobby. So I've decided to take my first stab at building a valve amplifier and figured, what better way than the ST-70, which seems to have the most documentation and support of others.

Any help or support you guys can provide is greatly appreciated, and I will certainly keep you posted on my progress.

I was looking around on eBay the other day and stumbled across a Tomiko toroidal PT that claims to be a direct drop-in (minus obvious mounting issues). Has anyone had any experience with this item?
 
You should do fine with your speakers connected to the 4 Ω taps.

Have you decided on a driver board? The OEM 7199 is scarce and expensive. FWIW, I like Triode Electronics' board, which retains Dyna's topology, while employing tubes that are in production. AAMOF, TE's board populated with 2X EF86es and an ECC99 will outperform Dyna's original.

"Exact clones" of Dyna's magnetics are available. I know nothing about the toroid you mentioned.

Current production 5AR4 rectifiers are a dicey proposition. If you like, I can provide a method for using SS B+ rectification that does not overly stress anything in the amp.
 
If you are set on a ST-70 clone, there are lots of places to pick up parts & iron. Here are a few among many:

There are quite a number of alternative driver PCBs for the ST-70 that use different tubes and rectification, so do a search for that here and on Google......

For ST-70 Dynaclone iron check out Triode Electronics:

Triode Electronics Tube Amp Kits Transformers Tubes Dynaco Upgrades and Parts

The Triode electronics PA-060S power transformer is a great choice, as it is rated for higher current than the stock unit, and it's beefier with more lams. The stock power transformer can be a little marginal, esp. if you want to run KT88's at some point.

And there is an entire forum dedicated to the ST-70 over at DIY tube:

diytube.com :: View Forum - st70

Can you provide a little more info on the Ebay toroidal transformer?
 

taj

diyAudio Member
2005-02-23 8:49 pm
Current production 5AR4 rectifiers are a dicey proposition. If you like, I can provide a method for using SS B+ rectification that does not overly stress anything in the amp.

Hi Eli,

I'd like to hear about your 5AR4 to SS method that keeps the operating points within reason. I've just seen BIG dropping resistors used with the SS diodes before, and I thought there must be a better way.

I'm going to use Gregg V.'s (Geek's) DynaMutt circuit in my ST-70.

Anyone know where I can buy 716P orange drops for a reasonable price? (Bonus points if it's in Canada).

..Todd
 
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I'd like to hear about your 5AR4 to SS method that keeps the operating points within reason. I've just seen BIG dropping resistors used with the SS diodes before, and I thought there must be a better way.

My method starts with a "stock" ST70 that has 1 bias adjustment per channel. I retain that arrangement, while dealing with minor variations in cathode current in an O/P pair. The fundamental concept is to turn the extra Volts SS rectification yields from a problem into an advantage. Insert a CL150 inrush current limiter between the rectifying diodes and the PSU filter. Select a 1st filter cap. just "tall" enough to keep the rail voltage satisfactorily up. Replace the crappy OEM choke with something actually useful. Pile the energy storage up after the choke.

The "secret" is combination bias. ;) "Stand" each pair of "finals" on a shared bias network made from a 100 Ω resistor and a 470 μF. 'lytic. A resistance of 100 Ω provides a convenient "idle" current test point and "eats" the extra Volts yielded by SS B+ rectification. The shared self bias network compensates for minor tube to tube cathode current variation and allows the single bias trim pot. to do a decent job. Close matching of gm is still necessary in each pair of "finals".
 
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Can you define "crappy"? My guess is we need a bigger choke, 5+H?

I'll see if I can find out how a combination bias circuit is wired. Any hints?

Does the CL150 slow things down enough to skip using a standby switch at start-up?

..Todd


Dyna's choke is not up to snuff, current handling wise. A genuine 250 mA. or greater capable part is in order. An inductance of 2 H. should be enough.

In an amp with "fixed" bias, any delay in B+ rise is enough for an "instant on" bias supply to provide electrostatic protection against cathode stripping. So, a standby switch is unnecessary, when the inrush current limiting thermistor is installed.
 
I'll see if I can find out how a combination bias circuit is wired. Any hints?


..Todd

It's the 100R cathode R's replacing the stock 15R (IIRC stock is around 15R....)....they reduce some/most of the extra volts across the tube from the SS rectification. It kinda looks like cathode bias but it's still fixed bias, or fixed bias with extra large cathode R's.
 
mixed bias

I used to use "mixed bias" on my guitar amp based on a Deluxe Reverb chassis. I switched to SS rectification and needed to eat up the extra volts. I always liked the clipping behavior of cathode biased guitar amps, so I thought I might like the mixed bias config.

On the scope, the clipping behavior of the mixed bias circuit was sort of like the very square, "flat-topped" clipping of a fixed bias circuit, but with noticeably rounded shoulders. Makes for a rounder sounding distortion in a guitar amp. Slightly less output power, though. I couldn't tell there were a few watts missing, but I sure liked the smooth, round overdrive.

Just a little anecdotal evidence to support Eli's idea.

-=|=-
 

taj

diyAudio Member
2005-02-23 8:49 pm
Is this what we're talking about? (see diagram)

How much capacitance after the choke? Lots? Like an extra 100uF ea.?

..Todd
 

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Is this what we're talking about? (see diagram)

Yes, that's it. Part of the total bias voltage comes from the drop across the 100 Ω resistor. The bulk of the bias voltage comes from the already present negative supply. Please notice that setting the operating point is easy. For argument's sake, make the EL34 "idle" current 40 mA. per tube. Adjust the bias trim pot. to get an 8 Volt drop across the 100 Ω part.
 
Quote:
Yes, that's it. Part of the total bias voltage comes from the drop across the 100 Ω resistor. The bulk of the bias voltage comes from the already present negative supply. Please notice that setting the operating point is easy. For argument's sake, make the EL34 "idle" current 40 mA. per tube. Adjust the bias trim pot. to get an 8 Volt drop across the 100 Ω part.

Please enlighten me... I don't see the advantage in installing a 100 ohm resistor in place of the stock sized resistor from cathode to ground . I have been told by some on this board that installing a smaller than 10 ohm resistor from cathode to ground would make the amplifier sound better. What exactly will make this mod out perform installing an additional bias pot with two ten ohm resistors (one per tube) cathode to ground and adjusting the bias to obtain 40mA per tube. The mod that has an additional bias pot would allow for any miss match that is likely to be there when dealing with used and or unmatched tubes. Having to deal with the original factory designed bias circuit is a pain.
 

20to20

Member
2010-06-23 9:25 pm
Is this what we're talking about? (see diagram)

How much capacitance after the choke? Lots? Like an extra 100uF ea.?

..Todd

The Power output of the '70 is rated at 35W continuous and 80W peak.
The FR is +/- .5db from 10 hz to 40 Khz
The Power response is 20 -20Khz within 1 db at 35W.

These are well recognized specs that have been accepted and respected, confirmable, part of the Dynaco reputation, yadda yadda....

How does adding 500% more filtering improve the specs in any audible or scopeable way? And what are the specs after the mod is done?
 
Quote:

Please enlighten me... I don't see the advantage in installing a 100 ohm resistor in place of the stock sized resistor from cathode to ground . I have been told by some on this board that installing a smaller than 10 ohm resistor from cathode to ground would make the amplifier sound better. What exactly will make this mod out perform installing an additional bias pot with two ten ohm resistors (one per tube) cathode to ground and adjusting the bias to obtain 40mA per tube. The mod that has an additional bias pot would allow for any miss match that is likely to be there when dealing with used and or unmatched tubes. Having to deal with the original factory designed bias circuit is a pain.

The "hybrid bias" scheme will allow the higher B+ voltage of SS rectification without exceeding the power dissipation limit of the output tubes. The 100R cathode bias R's will drop more volts than the stock 15R, reducing the voltage drop across the output tubes.

You could still use separate bias adjust pots (one per tube) with the hybrid bias scheme also......
 
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taj

diyAudio Member
2005-02-23 8:49 pm
The Power output of the '70 is rated at 35W continuous and 80W peak.
The FR is +/- .5db from 10 hz to 40 Khz
The Power response is 20 -20Khz within 1 db at 35W.

These are well recognized specs that have been accepted and respected, confirmable, part of the Dynaco reputation, yadda yadda....

How does adding 500% more filtering improve the specs in any audible or scopeable way? And what are the specs after the mod is done?

Eli said something like load up on the energy storage after the choke. My question was simply how much (with an uneducated guess). I certainly wasn't proposing a 'mod', just looking for guidance.

And regarding the ST-70 specs... I don't know anyone here who was discussing an original ST-70. The OP is using a KT-88 driver board, IIRC. I mentioned a different one for my rebuild. Thus, these are no longer Dynaco amplifiers, and the original spec's aren't a consideration at all.

What does concern me, is converting the power supply to solid state and doing it in such a way that the B+ voltages are compatible with the original tube complement and hopefully some common driver boards.

But primarily, I'm learning from the discussion, and I hope it continues, (despite me kind-of hijacking the topic somewhat).

..Todd
 
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20to20

Member
2010-06-23 9:25 pm
And regarding the ST-70 specs... I don't know anyone here who was discussing an original ST-70. The OP is using a KT-88 driver board, IIRC. I mentioned a different one for my rebuild. Thus, these are no longer Dynaco amplifiers, and the original spec's aren't a consideration at all.
Todd,
Gumby started the thread looking for advice about building the '70. All of it has been directed at the mods people do and installing high amounts of new capacitance is one commonly promoted. I thought that was something you had a question about.
 
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