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Old 13th Aug 2019, 9:19 pm   #28
ms660
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Join Date: Apr 2011
Location: Cornwall, UK.
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Default Re: Convert Vintage Valve Radio to Guitar Amp

Quote:
Originally Posted by robbo100 View Post

@ms660

In terms of replacing C19 - I presume something like this would be suitable (400V DC rated 0.1 uf/100nf).

Interestingly, R15 doesn't show on one of the versions of the schematic I have found, and it is listed as R13. But it looks like 270 ohm 1W. When you say "a suitably rated capacitor" - what would you advise. I have looked at some calculators, but am a little confused (including what voltage rating it should be).

Finally, in the event that I do create a buffer circuit to increase the input impedance, I was thinking of powering it using the lamp circuit, which is 6.5V. I need to see if I can find a suitable opamp that will work at this power (possibly the LM741?). Does this seem a sensible approach?
The original value for C19 is 0.02uF (0.022uF/22nF in todays money) If you want to reduce the value try 0.01uF or 0.0047uF or even lower, it's a case of what suits for the guitar sound.....400 VDC rated is ok.

The cathode bias resistor for the audio output valve is circuit ref R15 in the Trader sheet and R13 in the manufactures manual...270 Ohms in both cases, try a 22uF@50VDC for a cathode bypass capacitor, that will be an electrolytic capacitor and will be polarized, therefore it's -ve lead should connect to the chassis and it's +ve lead should connect to the cathode, pin 2 of V4, the audio output valve.

I'm not really qualified to comment on the suitability of any op amp (someone else will be) but if you do decide to power a separate pre-amp from the heater supply you need to be aware that, in this receiver, one side of the heater supply is connected to chassis which means you can't use a bridge rectifier for the supply with the heater supply as configured, if you do, something will go splat, so it will need to be half wave rectification only from the heater supply.

https://www.cricklewoodelectronics.com/

(Other component suppliers are available)

Lawrence.
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