I just received Triode Electronics EF86 input board for for the ST-70. The parts list specifies four resistors at 18K (R3, R6, R9, R15). Supplied resistors are 20K (not high measuring 18K, but actual 20K, as indicated by the bands). I have 18K resistors on hand. Should i use my 18K resistors or the 20K ones supplied?
Without a schematic it is hard to tell. I would ask Triode Electronics. Could be a misktake, or an “improved value” for the circuit.
Carly,
Is this the board you have, see pics? Probably doesn't make that much difference as it's only a 10% variance. Triode likely didn't have 18Ks on hand. Also of note on the board I have there is no connection between pin 8, the screen grid and pin 3, the cathode on V1 and V3. If yours is the same and you are using EF86s you'll need to make this connection.
S.
Is this the board you have, see pics? Probably doesn't make that much difference as it's only a 10% variance. Triode likely didn't have 18Ks on hand. Also of note on the board I have there is no connection between pin 8, the screen grid and pin 3, the cathode on V1 and V3. If yours is the same and you are using EF86s you'll need to make this connection.
S.
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Since these 18k resistors do different things at different circuit points, I might suspect that it was a case of needing to ship product and substituting something that was in-stock and would work, especially since there are all kinds of parts out of stock at major distributors these days.
Hi Steve,
Yes, i'll be using EF86s, but when i flip over the board there is already a trace between pins 3 and 8. Should be good to go it think. By the way anyone know who will ship EF86s quickly? I'm in Canada.
The manual calls these 4 resistors “Phase Compensation RC Filter”. If OP has them in stock I would be inclined to use the 18k value specified, rather than 20k provided (probably due to the supply problems already mentioned). However, OP paid good money for the kit, why not ask Triode Elec.?
Partsconnexion has an OK price on Electro-Harmonix and they usually ship quickly.
https://www.partsconnexion.com/TUBEEH-67234.html
https://www.partsconnexion.com/TUBEEH-67234.html
I wouldn't worry too much about the value of the resistors in the phase compensation RC filter (one per channel) being 20K instead of 18K.
In the original ST-70, which uses a 7199 instead of EF86 + 0,5 x 12AU7, the value is also 18K (and the value of the associated capacitor is 82 pF in both situations). What are the chances that the optimum values of this phase compensation resistor and capacitor are exactly the same for these two situations? And if the output transformers used don't have exactly the same specifications as the original output transformers, I would think that the chance that the phase compensation still works best with 18K + 82 pF even get's smaller.
Probably best is to find the optimum values using a scope.
In the original ST-70, which uses a 7199 instead of EF86 + 0,5 x 12AU7, the value is also 18K (and the value of the associated capacitor is 82 pF in both situations). What are the chances that the optimum values of this phase compensation resistor and capacitor are exactly the same for these two situations? And if the output transformers used don't have exactly the same specifications as the original output transformers, I would think that the chance that the phase compensation still works best with 18K + 82 pF even get's smaller.
Probably best is to find the optimum values using a scope.
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