New Member

Hi everyone, I'm new to vacuum tube amps but not to vacuum tube technology.

I recently purchased a Dynakit ST70 on eBay. It is somewhat "modded" (speaker connection receptacles, tubes changed, knurled knobs for bias adjustment, etc)

I know I'll learn a lot here as I am sure very member of this forum knows more about Dynaco amps (and vacuum tube amps) than I.

I wanted a vacuum tube (analog) system to check my speakers out. I design and manufacture speakers as a "day" job.

In the spirit of learning I will look forward to suggestions, tips, questions, etc.

Looking forward to being a member

Welcome. You will find this forum is very knowledgeable and diverse. That also means we do not always agree on many issues for (usually) good reasons.

Good luck with your amp project and I am sure you will enjoy the results mated to your day job work. One thing to decide on. Do you want to restore the amp to as close to original as possible or to modify/upgrade it? Both have their advantages and you will find a healthy discussion from this group on the merits of each. Two major items to investigate are replacement of all capacitors and adjusting the bias for longer output tube life.

Check out the photo section of this forum. Nice work by many of the forum. You have to leave the basket section to get there.

Yes, Welcome!

You will find a vintage ST-70 to be an interesting choice to drive speakers. Depending very much on the pre-amp you use, it may sound, for lack of a better word, Lumpy. The damping factor of the OEM 70 is around 15. For a (relatively) modern Solid-State amp, than number will be in the hundreds, anyway. 50 is typically considered as the minimum 'correct' number. With more qualifications, exceptions and opinions attached to that number than ornaments on a very busy Christmas tree.

What Damping Factor "is" - the ratio of speaker impedance to amp impedance.
What Damping Factor does - it acts as the brakes on a speaker preventing overshoot - most especially at low frequencies.

How to demonstrate - obtain a small DC PM brush-type motor - something for an old plastic model back in the day. Spin it with your fingers. Pretty easy? Sure it is. Now, short the motor leads. Spin it with your fingers - not so easy. That is how damping works on your speaker.

For small, low-mass speakers, this is not much of an issue. For older speakers with hard surrounds and tight spiders, also not much of an issue. Where this becomes an issue is with acoustic-suspended large woofers with soft surrounds and soft spiders. The mass of the cone creates enough inertia to overshoot if not controlled.

In any case, Enjoy!

Welcome, Cav Trooper! This well-moderated forum has more than once been a lifeline for me, and also the source of the occasional good belly-laugh.
Posters come from all levels of expertise and experience. Remember the old adage: "The only stupid question is the one not asked." Nobody here will flame you for not knowing something.

The very best objective analysis of the stock ST-70, IMO, was performed by Dave Gillespie. I would consider it required reading for anyone interested in knowing how the ST-70 actually functions and performs, from the techincal point of view, as well as its shortcomings. Here's a link to the article:

http://audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

Analysis of output impedance and damping begins about half way down page 6, if that's of prime concern to you, however, be sure to read the whole article. It's well worth the time it takes.

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It would be interesting to know what amplifier(s) you have been using to test your speakers in the past. This would serve to help set expectations for your Stereo 70.

The Stereo 70 can be a fun, hobbyist tube amp since parts and information are readily available. You noted that you are familiar with vacuum tube technology--so you know that potentially lethal voltages exist "all over" the Stereo 70, above and below the chassis.

You mentioned that the tubes had been changed. What tube types are installed? That will be a tip off regarding what circuit changes have been done to the amp.

...j

Welcome, Cav. Check out Sandeep's post "VTA120 speaker problem" . You may be of help. Dale

welcome to the tube audio abyss!!.......May your wallet be perpetually empty!!
On a somewhat slightly more serious note, there is nothing like listening to analog music...the way ot was meant to be!

Some great responses, questions, etc. Let me try to answer some of them.

I believe the Dynakit I purchased was only lightly modified. The bias setting screws were changed to knurled knobs, the rear speaker outputs were changed to banana plug receptacles, the chassis was highly polished, etc. I'll psot a picture on the photo side.

I believe there is a missing capacitor (you'll see what I mean on the photo.

It cane with 4 Electro Harmonix 6CA7EH output tubes. I was unable to get the bias voltage set (not even close). Looking at the Electo Harmonix and the different markings on them I think they were manufactured at different times with no thought to their values.

It came with 4 Tesla EL34's which seemed from the box markings to be somewhat matched, at least as pairs. When I installed them the bias voltage was easy to set at 1.56 volts.

It plays now, sounds good but again I think a cap is missing?

What am I expecting? Two things, listening to music simply as pleasure and being able to demonstrate my speakers to potential customers showing they have been played with a solid state integrated receiver (Denon) using AB amp technology, Class D amplification as well as vacuum tube amplification. The class D are both off the shelf and of my design (based on a TI amp module)

I agree with Mr Gillespie (great article, thanks) and for now I want this to be 'stock" and really see how it is to listen to and operate.

I'll head over to the photos and post a shot right after I hit send

I just looked at your photo and do see the adjustment knobs and missing capacitor. I also see a very polished chassis, a non stock rectifier, and a non stock circuit board.

If you really want it as close to stock as possible then you should take the published schematic (found on this forum) and go under the chassis and find/match all the parts.

A few comments on the amp. The chassis looks pretty good so the polish is a nice touch. All the lettering is gone but you have no rust.

The knobs on the bias adjustment are not needed but if you like them you could leave them. If they were installed over non stock adjustment pots then you should check the value of the pot out.

It looks like a 5U4 rectifier which will work. However the correct tube would be the 5AR4. If you search this forum you will find many discussions about this issue. Locate a 5AR4 and use it. It may change the bias settings so do not just plug and play.

When you were verifying the parts versus the schematic you will find out what happened to the missing capacitor. They probably substituted a number of axial caps underneath and soldered then in place. Make sure they are held/supported well and have the correct ratings.

The new circuit board looks to be a direct replacement but sometimes they use the 6GH8 tube not the 7199. Check out which one you have. If it is the 7199 then it is just a newer board but may have the stock circuit. It looks it from the cap values. It does have all new film caps and various types of resistors. You will find many discussions about resistor types and merits of each. The original ST 70 had carbon compositions.

Different output tubes can be fun to roll and do make a difference in sound. All have different costs and advantages.

Depending on what you find you may not need to do much at all to this amp. One important item is the bias setting. You will find support on this forum for turning the idle current down and not set the bias for 1.56VDC. Check your cathode resistors to verify they are each 15.6 ohms. If so then readjust the bias to 1.2-1.3VDC. This brings the output tube idle current down. It will sound great but gives much longer output tube life.

I ordered the appropriate rectifier tube and will put it in tomorrow, and also set the bias once again. While I am doing that I will go over the schematic once more and see what I can find.

WLT, Thank you

A brief update.

I did a lot more checking on my Dynakit and discovered two things

First, The unit had the Curcio Audio Engineering quad cap module installed.

When I put it on my bench for a more detailed examination that came to light and I reached out to Joe who confirmed it (I sent him some detailed photos)

Second, I discovered one channel had a weak output. By swapping leads around I found it was one of the 7199's. I used a couple of adaptors that Joe suggested and replaced both 7199's with 6U8A's all from Tube Depot. The amp now sounds good and has a good output form both channels

The amp, to me, sounds pretty good now. I don't have any other vacuum tube amps on hand to compare it to though.

Lot of great feedback and questions and as a new guy here I really appreciate all the assistance.