Firefly Power Amp Help

[johngovan]

I build a Firefly power amp using 12au7 tube.

Plate voltage                   232v
Cathode voltage drop      8.1v
Cathode Resistor           500R


Then i hook my fender preamp to the power amp. Which sounds good. But my problem is the 12au7 gets too hot when I touch it.

1. Is that normal or am i missing something?

2. Can i put a cathode bypass resistor in the poweramp section to make it louder?

[sluckey]

The tube should be hot. A bypass cap will improve the bass response and make the amp a bit louder. Try it. However, the main purpose of the firefly is to NOT be loud.

[johngovan]

The tube should be hot. A bypass cap will improve the bass response and make the amp a bit louder. Try it. However, the main purpose of the firefly is to NOT be loud.

Thank you for your help sir. I think this is normal. :) Another thing is. Can i hook 10 ohm Resistor in the output speaker to use headphones or make it as a line out and hooking it to a poweramp?

[PRR]

12AU7 is safe with glass slightly hotter than the boiling point of water.

You would NOT hold on to it at this heat!

Mostly we use these little tubes at small power. I have seen them run so not-hot that _I_ could touch them a few seconds, even pull them out of the socket live. (I have tough skin.) But the last tube in the Firefly works hotter than that. I would not put a finger on it. That's probably OK.

> Plate voltage                   232v
> Cathode voltage drop      8.1v
> Cathode Resistor           500R

Ah, numbers!! 16.2mA, 224V(p-k), is 3.63 Watts in two plates. Each plate is rated 2.75 Watts, or 5.5W for both together. You are working at 2/3rd of maximum. Hot, not too-hot. Will live a long time. (I had a similar amp with a lesser tube working slightly past rating, HOT. I had to replace the tube every year. But in those days this ex-TV tube was going out of style and could be got for a buck.)

I just tripped-over a Brimar report which includes push-pull power amp data for 12AU7. Yes, 0.7-1.1 Watts with <3% THD. IIRC the FireFly is not push-pull drive so will not be this "good"; but "baaad" can be good in a guitar amp. (3%THD not rising is really too clean for guitar expression.)
https://frank.pocnet.net/sheets/184/1/12AU7.pdf

[johngovan]

12AU7 is safe with glass slightly hotter than the boiling point of water.

You would NOT hold on to it at this heat!

Mostly we use these little tubes at small power. I have seen them run so not-hot that _I_ could touch them a few seconds, even pull them out of the socket live. (I have tough skin.) But the last tube in the Firefly works hotter than that. I would not put a finger on it. That's probably OK.

> Plate voltage                   232v
> Cathode voltage drop      8.1v
> Cathode Resistor           500R

Ah, numbers!! 16.2mA, 224V(p-k), is 3.63 Watts in two plates. Each plate is rated 2.75 Watts, or 5.5W for both together. You are working at 2/3rd of maximum. Hot, not too-hot. Will live a long time. (I had a similar amp with a lesser tube working slightly past rating, HOT. I had to replace the tube every year. But in those days this ex-TV tube was going out of style and could be got for a buck.)

I just tripped-over a Brimar report which includes push-pull power amp data for 12AU7. Yes, 0.7-1.1 Watts with <3% THD. IIRC the FireFly is not push-pull drive so will not be this "good"; but "baaad" can be good in a guitar amp. (3%THD not rising is really too clean for guitar expression.)
https://frank.pocnet.net/sheets/184/1/12AU7.pdf

So i can still bump the B+ to about 350v?

[PRR]

> So i can still bump the B+ to about 350v?

No. Maybe 284V, 300V if 12AU7s are cheap to you. Power output is raised almost insignificantly, less than 2dB.

[kagliostro]

With less voltage, may be you can give a try to this

https://tubes-store.com/product_info.php?products_id=15

Note the anode dissipation power

The 6N6P double triode is miniature device enclosed in glass bulb provided with a nine-pin base and an indirectly heated oxide-coated cathode and designed for low frequency power amplification.
The 6N6P double triode is resistant to ambient temperature from -60 to +70°C and relative humidity of 95 to 98% at +40°C, as well as to mechanical loads: linear loads up to 100g, vibration loads up to 2.5g, and single impact loads up to 500g and multiple impact loads up to 12g. Equivalent to the ECC99, similar to the E182CC. NEW, NOS and never used before!
Original name: 6N6P / 6Н6П;
Type: double triode;
Application: LF power amplification;
Cathode type: oxide, indirect heating;
Envelope: glass, miniature;
Filament voltage, V: 6,3;
Filament current, A: 0,59-0,81;
Anode voltage, V: 120;
Anode current, A: 0,02-0,04;
Anode power, W: 4,8;
Grid1 voltage, V: minus 2;
Steepness, mA/V: 8,1-13,9;
Reverse grid current, uA: 0,5;
Microphonic noise, mV: 100;
Gain: 16,0-24,0;
Socket type: rsh8;
Maximum weight: 20 gr;
Service life guarantee: 2000 hr.

Franco

[johngovan]

Thank you for the responses. I really appreciate it. I finished my roughly build. Then i power it on. And walla! There is a sound coming through. But my problem is when i increase the gain, it somehow act as a vibrato and as i increase the gain. The vibrato becomes slower.. I think it is motorboating as they called it. The OT is good. My filter caps are good also. I resolder everything and yet it has the same problem.

I don't know what cause the problem. I tried to disconnect the preamp from the poweramp section. And i test the poweramp. And it works even in full Volume. But when i connect the preamp. It begans to motorboat. I don't know why. Maybe my heater wires are to thin?

[SILVERGUN]

Do you have a schematic of your build?

[johngovan]

This one. Firefly amp.