st70 resistors

Can the resistors on an original ST70 board be measured for drift without removing them from the circuit?

rmk wrote:Can the resistors on an original ST70  board be measured for drift without removing them from the circuit?

Yes most of them
Exceptions:
330k ( 'A' to pin 6 7199)

The 1000 ohm overall feedback resistor which is in parallel with the 47 ohm (cathode resistor to the 7199) in series with the output transformer DCR. It will measure slightly less than 47 ohms or so.

Thank you both. I was wondering about those WLT.

But I think I have found a problem... The resistors supposed to be 620ohm. One side measured 657 ohm and the other side measured .924K ohm. Do I have a bad pair or is it not possible to measure those correctly?

rmk wrote:Thank you both. I was wondering about those WLT.

But I think I have found a problem... The resistors supposed to be 620ohm. One side measured 657 ohm and the other side measured .924K ohm. Do I have a bad pair or is it not possible to measure those correctly?

You have a bad pair. I recommend to replace both.
To be gentle with the board : cut the wires with a cutter, then heat and gently pull
the remnants one by one up to the component side. And use leaded solder ( 60/40 or so )

What is the function of the 620r resistors?

What prompted me to check resistors was some intermitent static in the right channel. Thats where the drifted 620r is.

Could it be the cause of noise?

rmk wrote:What is the function of the 620r resistors?

What prompted me to check resistors was some intermitent static in the right channel. Thats where the drifted 620r is.

Could it be the cause of noise?

The 620 ohm resistors make sure the 7199 pentode has correct bias.
A failing resistor ( especially the CC types ) that gets problem will be noicy.
I think 1/4w film resistor will be fine here.

Or, you could replace the board and take the opportunity to replace the 7199 tubes with
a pair of 6U8 or simular. See https://www.dynakitparts.com/shop/pc-3b-st-70-mf-resistors/

Is there anything else i should check because of the failed 620r resistors?

Btw despite the failed resistor, the amp sounds good (albeit the noise in the right channel) both channels sound like they have equal gain.

I also found that the 270k resistors in both channels have drifted... 291k/294k and 294k/284k ....
Does that warrant replacement?

rmk wrote:I also found that the 270k resistors in both channels have drifted... 291k/294k and 294k/284k ....
Does that warrant replacement?

You have 4 of them; the plate resistors these needs to be 1w film resistors that can take 400 volt.
The g1 resistors has less need for power but buying 4 ( or 5 ) is easier. These should be
the ones with closest tolerance.

1w?

those 4x 270k on the board were 1/2 watt, 10%, no?

Replacing the plate and cathode resistors for both sections of the 7199 is one of the best ways to improve performance of your amp with minimal cost. I use 2 watt carbon film types as they will fit on the circuit board just fine. The phase splitter needs 2 watt types anyway. PeterH (and many others) like the metal films. Both will be much better tolerance that the old carbon composition type that have now drifted over time. When you order either type they will come much better than 5 or 10%. The Kiwame type I usually buy are better than 2% always and most are closer to .5%. The 620 and 47 ohm can be 1/2 watt. The 270K is 1 watt. The 47Ks and 1000 must be 2 watt.

The 620 ohm resistor is in series with the 47 ohm to create the resistance from cathode to ground. Current flow creates the bias voltage needed for the voltage amp section of the 7199. The reason it is two separate resistors is the global feed back comes back thru the 1000 ohm to that junction point. The 1000 ohm and 47 ohm resistor creates a voltage divider that provide a precise amount of dB of feedback to this section. The 620 ohm, 47 ohm and 1000 ohm are all pretty important for these two reasons. Many people do not consider them significant but obviously Dynaco (Ed Laurant) did when they finalized this design. Merely look at the schematics of the MK II, III, IV, and VI to see the same topology. Maybe different resistor values but the same design.

I apologize for my ignorance as I do not know the difference between the functions of the 270K resistors.

The ones that drifted that I have are the 4x 1/2 watt ones, not the pair of 1W.

So do I need to up wattage and match those 4 when replacing them?

rmk wrote:1w?

those 4x 270k on the board were 1/2 watt, 10%, no?

1w will do for the plate resistors BUT they should be allowing 400Volt.
Larger powered resistors generally allows higher voltage but do make sure the ones you get
will stand against tuba amp voltages. Buy 4 to use in also the g1 resistors as they also drifted.

Ok, 1/2 w will do here. But again make sure thet can cope with 400Volt

So again (I apologize for my confusion), the 4x 270K are the ones that need higher wattage and matching?

I would have to place order, I think the resistors I have are rated 350v

Two of the 270 K ohm resisters are the grid resistors for the output tubes. Two per channel so four in the amp. The coupling caps will block DC so these only see negative DC bias voltage on them. That would be a maximum of 50 or 60 volts DC. Actually will have very little current thru them so almost no DC voltage drop across or thru them. AC voltage will vary across them with signal. They need to be matched to each other (2% or better) but they can be ½ watt. I still use 1 or 2 watt there but not required. I do not feel the voltage rating is important for these two. If the coupling caps fail you will have a much bigger problem than the voltage rating to these resistors.

The 270K resistor used for the plate resistor in the voltage amp section will have much higher DC voltage imposed on it. I agree with PeterH on the rating of that one. One per channel so two total in the amp.

thank you for the clear explanation wlt. I understand better now.

btw I recently replaced the coup. caps on this board which is why i am not just ditching the old board all together at this time.

Thanks wlt for the clarification. There is 2 270k used as plate resistors, these will need to
be 1/2w and able to work at 400Volt. Then there is 2 x 2 270k used as g1 resistors that needs less
power and spec. But it seems more convenient to buy 6 (or more) resistors and use at all 6 positions.

Some options for you
Kiwame specs
* Material: Carbon Film
* R.Range: E-24 class
* Wattage: 2W and 5W type, 750 volts DC max
* Error Ratings: +/-5% (actually +/-0.5%)
* Value Indication: Characters
* Insulation Material: Silicon Resin
* Body Color: Light Green
* Body Size: 2W type; D4.2 x L12 / 5W type; D9.0 x L24 (mm)

Available thru DIYhifisupply, Percy Audio or Parts Connection - all have minimum order points

a pack of 620 ohm here https://talonelectronics.com/shop/item.aspx?itemid=9821 watch for minimum order point
a pack of 2 watt 270 K on Ebay - https://www.ebay.com/itm/270K-Ohm-2-Watt-5-Carbon-Film-Resistor-10-Piece-Lot-CFR200J270K-RC/401007321139?hash=item5d5de62033:g:AqMAAOSwo6lWFa2B

The last one you can then match pairs with ten to choose from. If you want more sources for other ones let me know