New Rocket build

[andresound]

Voltages were a bit high with the custom wound mains transformer, So decided to throw in a 5U4G NOS tube just for kicks. Couldn't stop playing her. WOW!! Sounds fricken awesome. Haven't checked how much the voltage dropped by yet, and still have to check out bias resistor and measure dissipation.

But beautiful sound. Harmonic feedback even when playing clean!! Could not be happier. Will be interesting to see what the voltages have done.

[martin manning]

Indeed it will be interesting to see where the voltages are now that you have it singing. The 5U4 probably drops an additional 20V re GZ34, and it will add some sag. Something around 290V is the target for the plate voltage. Don't forget 5U4 needs an additional amp of filament current.

Accurate plate dissipation calculation means you need plate current alone, which you can get using the OT primary resistance, or you can use the cathode and screen resistor values to find Ia from Ik - Is.

[andresound]

Hi Martin,
I will check and post voltages. Here is a pic. Btw, The "motor boating" is still a mystery! Put the valves back in, and the problem came back. Took the offending set of NOS MULLARDS to a friend with an AVO tester, and they all tested good and 100% +!!. Strange

The PT has a 5v rating of 4A

[andresound]

Cabinet waiting to be made Red Rivergum. Extremely hard wood.

[andresound]

Indeed it will be interesting to see where the voltages are now that you have it singing. The 5U4 probably drops an additional 20V re GZ34, and it will add some sag. Something around 290V is the target for the plate voltage. Don't forget 5U4 needs an additional amp of filament current.

Accurate plate dissipation calculation means you need plate current alone, which you can get using the OT primary resistance, or you can use the cathode and screen resistor values to find Ia from Ik - Is.

Voltages briefly with the 5U4 installed (on 280-0-280 tap) are,
Cathode to plate of 286.4v
Voltage drop over cathode resistor (66 ohm) of 11.02
Plate (pin7) of 294

According to the tube bias calculator,

39.5ma per tube
11.3 watts per tube
= 94.2 % dissipation

I tried the MULLARD GZ34 again (on 250-0-250 tap)
41.9ma
12.5 watts
104.2 %
Plate (pin7) of 311.2

Although the higher dissipation (GZ34) sounds "bigger/bolder" and louder, I prefer the sound of the 5U4 installed. Seem to bloom and is rich in harmonics!!

[martin manning]

Looks great, and thanks for the report on the 5U4 vs. GZ34. What bias calculator are you using? For the 5U4, cathode current = 11.02/66/4 = 41.7 mA/tube, so I would estimate (294-11)*41.7 = 11.8W including screen current, maybe 90% of that without.

[andresound]

I used the Weber and the rorobinette.com bias calculators. They both gave the same result. However, I trust your calculations better . Also that means my bias would be sweet @ 11.8w.

FWIW, All rectifier tubes are NOS. I also have 2 Phillips GZ34 (made in Holland) and 2 other old 5U4's which I can't remember the brand, (all tested 100%+). I will do some tube rolling and see if the "rich harmonic" sound is tube dependent or voltage dependent.

[martin manning]

Thanks for the vote of confidence! As a rule I do not use on-line calculators as you often don't know what assumptions are being made. With 11.8W calculated using cathode current, you can assume actual plate dissipation (plate current x Va-k) is about 10% less.* Looks like the 66R resistor will be fine with either rectifier tube. I'm interested to hear what you find out re different brands of rectifier. Hi-Fi guys think it matters, but...

* For pentodes; beam types 5% less.

[andresound]

Hi Martin,
Will report this week on different tube brands. Will try the amp in a band situation tonight. BTW, I am a Hi-Fi guy. But, I have been to counselling and I'm OK now

[andresound]

Thanks for the vote of confidence! As a rule I do not use on-line calculators as you often don't know what assumptions are being made. With 11.8W calculated using cathode current, you can assume actual plate dissipation (plate current x Va-k) is about 10% less.* Looks like the 66R resistor will be fine with either rectifier tube. I'm interested to hear what you find out re different brands of rectifier. Hi-Fi guys think it matters, but...

* For pentodes; beam types 5% less.

Will the 66R resistor + GZ34 on 280-0-280v tap also be OK? I don't get red plating, so maybe it is. Just don't want to destroy a set of tubes. BTW, if the output tubes fry, do I risk damaging the OT?

[Bob S]

Good stuff. After some Rk rolling I settled on 68r with Va at 302 with a solid state full wave bridge. Also tried drinking with the PI drive amplitude. Think it ended up back at stock values.
I like the fact that tube rectifier type can be used to change the B+ value, but I must admit that I prefer a ss fwb with plenty of current capability in the power transformer. That might be more due to a reliability feel good factor than a sag/sonic thing.
Output tubes are a consumable - especially with a Rocket. I've never had an OT problem with them though.

[martin manning]

Will the 66R resistor + GZ34 on 280-0-280v tap also be OK? I don't get red plating, so maybe it is. Just don't want to destroy a set of tubes. BTW, if the output tubes fry, do I risk damaging the OT?

If you don't see red-plating at idle, you are probably ok. You will shorten the life of the tubes if you run them hot, though. If you're interested, calculate the dissipation to see how hot. If it's much over 100%, then it would be better to avoid that combination or increase the cathode resistor value.

It is possible that a shorted output tube can destroy an output transformer and/or other components before the mains fuse blows. The best insurance against that is to replace the output tubes before they fail, say after a year or two of regular use.

[andresound]

Thanks guys.
Martin, how much over 100% dissipation is too much?

[martin manning]

Measuring with cathode current I wouldn't run above 110%. That means actual anode dissipation would be ~100%. Calculate as follows:

Vk (Va - Vk)
-------------- should be <= 1.1
4 Rk Pa max

[andresound]

Been a bit quiet here, so decided to post. Finally... Almost.... Nearly... Finished cab.!!!! Just need to engrave faceplate. Might also consider a bright switch..... Tried a 4.7uf cathode bypass cap on second stage (didn't have a 22uf on hand)... Really like the sound.