Is it safe to use 6L6 in my amp.

[Alan0354]

OK Hopefully it's the last question about my amp: I just put in a pair of 6L6GC

1) The +B is down to 412V Max.
2) I adjusted the idle current to 35mA ( stay on the low side for 6L6).
3) I connect the 8.33 ohm load to the 8 ohm tap ( I swear, it is the real 8 ohm tap this time.)
4) I run 1KHz and increase the volume so the output clips. The clipping voltage is 45Vpp
5) From calculation, the output power is about 30W.

I want to see whether the PT can reliably handle the 6L6. As you know, I use the Weber W025130EU. https://taweber.powweb.com/store/025130eusch.jpg. I am using the Vibrolux output transformer that is spec 40W, so I am good with the OT here.

The PT is spec 150mA, max 25W. But I am using SS rectifiers and the 5VAC winding for the floating filament is only used to power the two relays and the HV opamp. The total current is about 120mA max. So I saved at least 5W right there. I have 3 12AX7 less than the Deluxe reverb because I don't have reverb and tremolo. So that is saving of power on the 6.3V winding also. So do I have anything to worry?

Thanks

[jazbo8]

How do you get 30W output from a PT rated for 25W?! Assuming that's a typo, the peak current of the 6L6s with your operating conditions would be over 180mA, which is still over the PT's rating. So I say worry...

[Alan0354]

How do you get 30W output from a PT rated for 25W?! Assuming that's a typo, the peak current of the 6L6s with your operating conditions would be over 180mA, which is still over the PT's rating. So I say worry...

Thanks for the reply.
The transformer spec for 25W. I am hoping to push to 30W since I use less on the filaments.

I have more to worry. The MOSFET I use for power scaling gets hot enough when is set to the lowest setting and run to clipping with the signal generator. The power dissipation is high because the MOSFET has to drop 300V across and source current to drive the load. I measure a 50mVpp across the 1 ohm resistor from the cathode to ground. With two push pull tube, it is close to 50mA DC. So power dissipation is 300V X0.05A=15W. The heat sink is touchable hot.

Maybe, I should go back to 6V6 after all.

[jazbo8]

Yes, I think 6V6 would be better suited for your current PT & OPT. Also for your crackling sound, have you tried it without the NFB connected?

[Alan0354]

Yes, I think 6V6 would be better suited for your current PT & OPT. Also for your crackling sound, have you tried it without the NFB connected?

Yes, I already go back to 6V6. I am testing the power output, and the power dissipation on the MOSFET. At +B=200V, I have power dissipation of 18W on the MOSFET. I am not even comfortable with that. I might still have to lower the +B a little.

The cracking sound was.....AGAIN.....my mistake. I was testing it late last night and I had the attenuator at high attenuation. Since the cracking sound happens when I crank beyond 7 at at different power setting, I thought it must be the amp. This morning, I put the attenuator at low attenuation or no attenuation and play loud, I did not hear the cracking anymore.

[jazbo8]

Yup, that happens to me too, when you are tired - you make more mistakes. Glad you sorted out the crackling issue, how do you like the Hotplate compared to the VVR/MOSFET control?

[Alan0354]

Yup, that happens to me too, when you are tired - you make more mistakes. Glad you sorted out the crackling issue, how do you like the Hotplate compared to the VVR/MOSFET control?

VVR definitely better. I yet to really test it, but on the first pass, it sounds very close at 10W, I can't tell the difference. At 5W, it's slightly off only. At 3W, I think it still sounds good. All in all, I am happy with the amp.

The Hot plate definitely change the sound even at -4dB. It is not bad. At -12dB, it's not very good. At low power, the VVR kicks the butt of the THD Hotplate.

[JoeM]

Alan, isn't the 25130 the PT for a Deluxe Reverb? There's quite a few people that use 6L6s in the DR.

http://music-electronics-forum.com/t34302/

[Alan0354]

Alan, isn't the 25130 the PT for a Deluxe Reverb? There's quite a few people that use 6L6s in the DR.

http://music-electronics-forum.com/t34302/

I read the whole thread, interesting. Looks like they keep the power down by mismatch the tube. I am using a Classic Tone Vibrolux OT that has multi tap outputs. So far I use the 4 ohm tap to drive the 8 ohm speaker to get 8K primary for the 6V6. When I switched to 6L6, I did connect the 8 ohm load to the 8 ohm tap and got 30W. I was afraid, so I switch the tube back.

I tried 6L6 with mismatched load. I only got 22.8W, just 1.2W extra over the 6V6. I did the calculation of the power dissipation of the MOSFET, it is up to 4W higher. That is reaching 20W on the FET. I am not comfortable with that. So I think it's not worth it for me to gain 1.2W.

Thanks



Alan

[Alan0354]

Thanks guys for all the help. I finalized using the Weber 6V6 and gets 21.6W.

This is really a learning process. This is the first time I ever design and build an amp from ground scratch. The ones in the pass were all using existing amp, using the transformers and build in my circuits. I mostly using the existing power amp. This time is like ripping everything out and put in all new stuff. The only thing from the original amp is the choke!!!

It's one thing studying tube books and tube theory, it's a totally different thing to put in in practice, to worry about output power, stage input output matching and all. I actually put load line into practice ( that I studied and forgot, only reminded by JMF ). Actually going through all the power calculations at different VVR setting and all. Then the Weber PT problem.

The amp is going into the combo cabinet today, just cleaning up, blow out the loose solder, wires from the chassis, double check the solder, tie up the wires and I am done!!! I tested the sound, I am very happy, even a 1W setting, it's quite good.

Thanks