A few ST-70 Questions

I have a few questions.

1. I've read countless times that people increase the .1 caps to .22 and the .05 caps to .1. And that this generally "improves" the sound. But I can't find any detailed description of what changes. Could someone please let me know the sonic changes would occur if I were to up the capacitor values? More/less bass? More/less treble? What about he midrange? Etc...

2. I really like the sound of my stock ST-70. However I haven't replaced a single capacitor or resistor on the driver board. And i'm sure most if not all the caps and resistors are out of spec. Most people seem to replace the WORN OUT driver board with a VTA or something similar. And almost always say its better. I'm curious though, can anyone here describe the sonic differences between a newly rebuilt STOCK ST-70 and a VTA?

           I'm curious about this as I'm kind of considering replacing the caps and maybe the resistors. At which point, perhaps a whole new board is in order. May I would build a new Stock version, or maybe something different like the VTA.

(FYI, I have replaced the selenium and the bias caps.)

I love this forum. Can't tell you all how many hours i've spent reading here. Thanks!

Thanks so much for your response! I really appreciate it.

However, I'd still like to know what difference I would HEAR upping the cap values .1 to .22 and .05 to .1.

And I get the technical differences of the VTA vs Stock. But I still don't know what the difference in sound is.

I think I'm leaning towards getting a new board from dynakitparts.com and doing a full stock replacement. But not sure if i want to up the 6 cap values. Thats why I'm wondering the the difference in SOUND will be.

The only way to know for sure is to swap them and listen for yourself.

Most certainly. I just figured since it seems to be quite common for people to up the values of those caps, that some people could chime in here and share their experience. I'll probably just stick with stock values and go from there.

Sorry, one more question.

I'd like to replace the 15.6 ohm resistors with something more easily available. I've read that 10 ohm 1/4 1% should work well. And that the low rating would allow them to act as a fuse. Is this correct?

Also, I currently bias the amp at 1.25v using the stock 15.6 resistors. Where would I bias the amp when using 10 ohm resistors?

Thank you!

I=V/R

With 1.25V across your 15.6 Ohm resistor your output tubes are biased for about 80mA (40mA each if your output tubes are matched for plate current).

If you wanted to run the same bias current after replacing the 15.6 Ohm resistor with 10 Ohms you would want to set Vbias=I*R=0.08*10=0.8V.

Thank you!

I just ordered a new stock board and upgraded quad cap, along with all new caps and resistors from dynakitparts.

Is the upgraded quad-cap wired the same as the original cap? And do I put 22,000 and 6800 resistors on as well?

.

Thanks!

Yeah the bias resistors were are both off by a few ohms. So they certainly need to be replaced. I have ordered replacements for every capacitor and resistor in the entire amp.

Well, i got the new STOCK ST-70 board up and running. Along with the new 40/80/30/20 cap. Sounds great! Also replaced the bias circuit and a few other resistors on the underside.

Also, I finally took a listen to my PAS-2 with the tone controls removed. Sounds so smooth and natural now! I'll keep listening this way, but I wanted to keep the options of using the tone controls, so I put them on a switch to turn them on or off. PERFECT.

The whole system is sounding awesome with my early KLH Model 6 pair!

Thank you all for the help!

I never had much luck with original sockets. YMMV.

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PeterCapo wrote:If you have the quad cap with the larger values, it should be wired in this order: 40/80/30/20.

Peter,
   Could you explain the rationale for this recommendation?

Thanks,
DAvid

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Kinski wrote:Thanks!

Yeah the bias resistors were are both off by a few ohms. So they certainly need to be replaced. I have ordered replacements for every capacitor and resistor in the entire amp.

Coming in late to this, however:

Before I trashed these resistors, I would triple-check my VOM to make sure that what I am seeing is the actual reality. I keep a range of test-resistors - precision resistors purchased specifically for the purpose. Far too often, I have seen weak batteries or other problems with the VOM causing misreads.

Same reason, I keep calibration tubes for the tube-tester.

PeterCapo wrote:My understanding has been as follows... please feel free to add additional rationale.  

First section, 40µF, node "D"
The 5AR4 rectifier has a datasheet maximum of 60µF for an input filter, so the 40µF section typically goes right after the 5AR4.  . . .

Thanks very much. This helps clarify things, including how the order you specified relates to the circuit.

David