• 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.

Fuse vs Transformer

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anatech,

If the Dyna Stereo 70 was not a good amplifier, I guess very little is lost if the amplifier is modified from the original. I am sorry I did not make it clear that the 1.5H choke was the only electro-magnetic part that was not re-used (except during my early modifications of the amp).

I was not aware that people thought the Dyna Stereo 70 was not a good amp (even as stock). In Lynn Olson’s book “Soul of Sound”, he relates that someone brought a stock Dyna Stereo 70 to an amplifier shoot out with one of the latest (at that time) super solid state power amps. At the shootout, a soldering iron and 10 minute modification was done to that Stock Dyna Stereo 70. The modified Dyna ran circles around the solid state amp, and the group was never invited back (for showing the tube amp could blow away the whizzbang SS amp). Maybe that is why some of us like tube amps (even if we also like SS amps). And maybe simple modifications are worthwhile sometimes.

The brand new transformers I purchased for other amplifiers were not cheap. True, the incidence of shorts was few and far between, and the bad ones were replaced for free.

You mentioned that many homes in your area do not have 3-wire power outlets. That can be quite a problem, and may require different treatment of amplifier power and chassis connections. It also may restrict which kind of products can be used with each other, when they have different power connection and chassis connection methods.

I did not understand your words: “You should see what musicians do to each other. That would impress you”. Can you enlighten me please? Thanks!
 
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Hi 6A3sUMMER,
The Dynaco is easily bested by any number of other tube amps, and I have several solid state amplifiers that are a big improvement. The tube guys like these amplifiers too. The reason they are popular is that the market was flooded with these inexpensive amplifiers. The chassis is too small, as are the output transformers. If you perform the best mods to an ST-70, do you know what you end up with? An Eico HF-87.

In older areas in any city you will find mostly 2 wire outlets. By converting a piece of equipment you are causing hardship to those in that situation. In the best scenario they will use a three wire to 2 wire (plus a flying lead) adapter to plug them in. Mostly they just cut off the ground pin leaving the amplifier in at least as bad a state as it was coming in. You really are much further ahead by leaving the AC wiring the same as when it came in. Replace cords, fuse holders and transformers as required, but don't change the way it was wired. I was tempted to ground everything much earlier in my career until an older service person set me straight. I hadn't actually done this, but the conversation with a few of my mentors showed me the error of that kind of thinking.

Repair guys in the rock world to unspeakable things to instrument amps and anything else they touch. It has been known that if the band was extremely upset with the venu, they have rewired stage outlets to be 220 VAC. Small bars, not the large ones. They also have an element that will steal equipment and instruments from each other. So if you repair musician's products you will often have no budget for the repair, and have to undo a lot of hack work before you can actually fix something. Beware of cheap and incorrect tubes! You haven't lived until you see a 5881A arc in a Fender Twin of some flavor. Many think a 5881A is a substitute for a 6L6GC, which isn't true.

Best, Chris
 
Hi anatech,

The Eico HF-87 schematic looks pretty good. I prefer its cathode coupled phase invertor over the Dyna concertina phase splitter. The Dyna input pentode was pretty wimpy too. And there is about 9 dB more global negative feedback in the Dyna, it might be nicer with the lower feedback level the Eico has; and the Dyna has an UL screen capacitor feedback pole too. In spite of all the Dyna’s weaknesses, I did enjoy listening to the Dyna.

For my phase splitters, I prefer to use a ‘real’ current source to run the coupled cathodes. I never wanted to use a resistor for the cathodes, because you have to have the correct different values of the plate load resistors, or adjust them if the triode pair is not well balanced. Well, negative feedback can help make up the distortion that is a result of the phase splitter imbalance. We do have lots more options of how to (easily) build a current source than we had when the HF-87 was designed.

I remember the adapters that converted from 2-wire outlets to 3-wire plugs. They always made me nervous. Many users just leave the flying lead on the adapter hang in free space, or cut it off. And I believe many homes with 2-wire outlets might not have brought the ground up to the metal outlet box. In that case, connecting the flying lead to the screw that holds the plastic cover plate to the metal box does not do much (other easing someone’s mind into thinking it is safe).

Yes, old products, and new products; old cords, and new cords; old outlets, and new outlets presents a problem when they are all tied together in a single Hi Fi system.

I was not aware of any of the tricks you mentioned that musicians play (I may have been better off not knowing, but thanks for the education). I do know that guitar amps have those who roll tubes without knowing what they are doing (and so do some Hi Fi types too). We should “screen” those people from using a tube that either can not take the applied screen voltage, can not take the plate dissipation, or both. Yes, 6L6 and 5881 are bad examples of what to plug into a 6L6GC socket.

For restoration, the path you are taking is correct. On the other hand, I do change things. But I do so with the fact that the whole system, including the power outlet, signal sources, power amp, etc. are all taken into account, are working well together, and are safe to use that way.
 
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Hi 6A3sUMMER,
Fair enough.

Any new build I do is a three wire cord, wired properly of course. But when dealing with an older product you really should maintain whatever type of "grounding" it used from the start. My old HF-87 is still 99% factory, except I run the 12AX7A off the heater winding instead of using it as a cathode resistor for the outputs.

I may add regulation to an older audio component, but otherwise the signal path remains unchanged due to the risk of changing the sound away from what it was. I try to be careful about that.

-Chris
 
Hi anatech,

That is nice. You make it so your customer gets the same sound he had before. He just wanted you to make it last many more years.

Then there is that person who says: when I was real young and had my first Hi Fi, I really enjoyed listening to that [. . . brand name] amp on my speakers. I should not have sold it. Now I found and purchased a restored [. . . brand name] amp.

I do have a nostalgic streak in me. But I also have another streak that drives me to try new combinations of amplifier circuits.

I once had an amplifier with 12AX7s in the phono input that used the output tubes current to power the preamp filaments. It had a few problems, but I sold it before I could deal with those (I told the customer about the problems). It was listenable, but could have used some judicious work.
 
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Hi 6A3sUMMER,
I do have customers who want the distortion lowered. They sometimes retain the original sound character, but cleaner. Other times the sound changes (like a Pioneer) and is much cleaner. It really depends on what the customer wants.

-Chris
 
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