New Rocket build

[martin manning]

Start slow... no tubes, power up and check PT primary and secondary voltages; add rectifier and check DC voltage, add power tubes, check voltages, etc.

[andresound]

Powered up, no tubes with lightbulb limiter, no smoke. Powered up without limiter, no tubes, and it blows the 3A fuse! This transformer is huge (AC30 spec), maybe fuse is too small.

300ma
6.3 @ 8 amps
5 @ 4 amps

Will check wiring this evening

[andresound]

Fired up amp. Needed to adjust cathode bias resistor to stop red plating. Thank you Martin for your advice!

Plugged in guitar. Amp sounds great and quiet, but once warmed up, I get a "thumping sound" about 1 second intervals!! It is quiet loud, so don't want to "cook" anything.

Any ideas??

[martin manning]

Some kind of low-frequency oscillation, likely in the power supply. How are the voltages looking now? Can you measure and post a full set? What value is the cathode resistor now?

You could also try disconnecting the power stage cathode bypass cap to see if that is the source.

[andresound]

Is this what you guys call"motor boating"?

[andresound]

Will remove the cathod bypass cap. Why do some layouts/clones not have bypass caps here?

Thought it sounded like a cap discharging type thing.

Cathode resistor is 85 ohm and translates to 85% dissipation. 312v cathode to plate v. Could not source a single cap with the values you suggested.Used a 33 ohm 25w in series

Will post set of voltages

[martin manning]

Yes motor boating is a LF oscillation, usually more than 1Hz, though. You will need the cathode bypass cap to get full power. I suggested disconnecting it in case it is charging up and then shorting. 85% is a bit low, so you'll probably want to find a smaller resistor eventually.

[andresound]

Turns out the set of output tubes was the cause!! Changed to a matched set of EH and no more "discharge" type sound.
Still need to roll preamp tubes etc.

Voltages are
AC=548 (off 250 tap. Still have 280 tap on a switch, but haven't tried it)

B+1 = 330
B+2 = 319
B+3 = 206.7
B+4 = 229.6
B+5 = 215.5

V1
P1 = 146.2
P3 = 1.091
P6 = 100.4
P8 = 0.782
V2
P1 = 229.3
P2 = 145.1
P3 = 145.5
V3
P1 = 151.6
P2 =31.3
P3 = 46.9
P6 = 159.1
P7 = 32.2

V 4,5,6,7,
P2 = 0.052
P3 = 12.53
P7 = 320.8
P9 = 318.0

I don't understand why B+3 would be lower than 4 & 5

[martin manning]

I was thinking it could be a coupling cap too, with an internal arc creating the noise at 1 sec intervals. Glad you found it.

Yes B+3 looks odd. check the power supply string connections carefully.

[andresound]

Yip, supply string was wrong

Well, thank you to everyone for their help, Maritn, Rj and Mark Fowler!!! I will now put some time on her and fine tune and report back.

LONG LIVE TAG!!!

[martin manning]

Great!

Re the bias point:
Cathode current 12.5V/85R=0.147A, 0.147/4=0.037A/tube
Screen curent (319-318)/100R=0.01A
Plate current 0.037-0.01=0.027
Va-k=320.8-12.5=308.3
Pa=308.3V*0.027A=8.32W
8.32/12=69%

You can reduce the Rk a bit.

[andresound]

Thanks Martin,
You clever guys crack me up!! Just to keep you on your toes, LOL!!!, I flicked the switch to 280 primary. I think I prefer the 250 tap and this is what I now get.

Voltages are now. (For dissipation calc)

AC=607
Cathode to plate = 340.2
Cathode to ground = 14.01
Plate (pin 7) = 350

[andresound]

FWIW, amp seems to prefer Tele rather than Gibson SG

[martin manning]

Except I made an error! See above. With the voltages above (280 tap) I get 10.6W, or 88%, assuming the same screen current (need screen and B+2 voltages to get that exactly).

[andresound]

B2 = 351.2
B+1 = 361.5
V4 pin 2 = .055