Hi All!
This is my first post since joining, excited to be here! I'm an almost total tube nube, and as my first real project I'm undertaking to build a low-power guitar amp out of junk box parts. I have collected up a few 6K6's and a single 6V6, a couple 12AX7's and a few other 12xx7 dual triode types. I found a 10-watt PT that gives me around 365 rectified/filtered and unloaded B+ which drops to around 304 volts when tied to a 6V6 plate through a 5k-ohm OPT primary (calculated base on approximately 18:1 turns ratio and 16-ohm load). I have a 1 K-ohm cathode resistor which gives me 23 mA cathode current, well within spec for the 6k6GT (and presumably for the 6V6 as well since its a higher-watt tube, but I didnt find a cathode current rating for 6V6). I should point out that my OPT is actually a 117 volt to 6.3 volt filament transformer since I don't have an actual OPT, and the turns ratio is based on the 117 to 6.3 volt spec printed on the OPT. I'm using the info found at https://www.radioremembered.org/outimp.htm to calculate the OPT impedance. My screen grid voltage is currently at 294 volts, which does seem to be higher than spec'ed max for a 6V6. The screen is B+ put through a 10K resistor.
I did plug in a 6k6GT long enough to measure 21 mA cathode current and 280 screen volts, with 307 volts on the plate. I powered down after measurements to avoid possible 6K6 tube overload until I learn more and ma sure I'm within specs for both 6v6 and 6K6
My basic question is this: given the B+ and cathode current data above, is a 5K transformer too low for either a 6K6 or 6V6?
This is my first post since joining, excited to be here! I'm an almost total tube nube, and as my first real project I'm undertaking to build a low-power guitar amp out of junk box parts. I have collected up a few 6K6's and a single 6V6, a couple 12AX7's and a few other 12xx7 dual triode types. I found a 10-watt PT that gives me around 365 rectified/filtered and unloaded B+ which drops to around 304 volts when tied to a 6V6 plate through a 5k-ohm OPT primary (calculated base on approximately 18:1 turns ratio and 16-ohm load). I have a 1 K-ohm cathode resistor which gives me 23 mA cathode current, well within spec for the 6k6GT (and presumably for the 6V6 as well since its a higher-watt tube, but I didnt find a cathode current rating for 6V6). I should point out that my OPT is actually a 117 volt to 6.3 volt filament transformer since I don't have an actual OPT, and the turns ratio is based on the 117 to 6.3 volt spec printed on the OPT. I'm using the info found at https://www.radioremembered.org/outimp.htm to calculate the OPT impedance. My screen grid voltage is currently at 294 volts, which does seem to be higher than spec'ed max for a 6V6. The screen is B+ put through a 10K resistor.
I did plug in a 6k6GT long enough to measure 21 mA cathode current and 280 screen volts, with 307 volts on the plate. I powered down after measurements to avoid possible 6K6 tube overload until I learn more and ma sure I'm within specs for both 6v6 and 6K6
My basic question is this: given the B+ and cathode current data above, is a 5K transformer too low for either a 6K6 or 6V6?
For most pentodes, optimal load is close to the DC supply voltage divided the anode current Ib. It gives you the DC tube resistance, not its much higher anode resistance ra.
Power transformer lacks a small air gap in the core to avoid saturation by the anode DC resistance. This does't warn the tube nor the trafo, but increases distortion. If you want it to guitar amp, it is said that distortion is a feature, not a problem.
Please, refer to:
https://ax84.com/archive/ax84.com/static/rdh4/chapte13.pdf
Take a reading at page 13 and the following.
Power transformer lacks a small air gap in the core to avoid saturation by the anode DC resistance. This does't warn the tube nor the trafo, but increases distortion. If you want it to guitar amp, it is said that distortion is a feature, not a problem.
Please, refer to:
https://ax84.com/archive/ax84.com/static/rdh4/chapte13.pdf
Take a reading at page 13 and the following.
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@ kodabmx,
Thank you for your reply! I already have the amp wired for a pair of power tubes in parallel, pin-for-pin, sharing the cathode resistor. I think the power supply will handle it without running too hot, especially if I go a bit higher on teh cathode resistor. I'll lose probably lose some volume, but I can live with that. On the other hand, some power supply sag might just change the tone in a good way, so its something I'll experiment with. My main concern is will a single 6k6GT handle the 5k-ohm OTP as long as plate dissipation doesn't exceed about 4 watts. I suppose I could insert a low-value hi-wattage resistor between B+ and the 6K6 plate to be safe.
@Osvaldo de Banfield,
Thank you as well for your reply! I will take a look at the article you linked, And I do not think some added distortion will be a problem especially since my end goal is to just have it work properly within the limits of my available junk box parts. In my own line of work as a technician I tend to "make it work first, look pretty second".
Thank you both again!
Thank you for your reply! I already have the amp wired for a pair of power tubes in parallel, pin-for-pin, sharing the cathode resistor. I think the power supply will handle it without running too hot, especially if I go a bit higher on teh cathode resistor. I'll lose probably lose some volume, but I can live with that. On the other hand, some power supply sag might just change the tone in a good way, so its something I'll experiment with. My main concern is will a single 6k6GT handle the 5k-ohm OTP as long as plate dissipation doesn't exceed about 4 watts. I suppose I could insert a low-value hi-wattage resistor between B+ and the 6K6 plate to be safe.
@Osvaldo de Banfield,
Thank you as well for your reply! I will take a look at the article you linked, And I do not think some added distortion will be a problem especially since my end goal is to just have it work properly within the limits of my available junk box parts. In my own line of work as a technician I tend to "make it work first, look pretty second".
Thank you both again!
@dtbradio,
As long as you manage the K6K plate+screen current (and thus dissipation) it should be OK with the 5K transformer load. However, distortion may be higher than you like due to the below-ideal load.
But do heed Koda’s caution regarding using an ungapped power transformer as a SE output transformer. IMO this is your bigger concern.