B+ too high

[ehedwr]

I have just replaced my GZ34 rectifier tube on my Epi Blues Custom 30 with a Sovtek solid state rectifier. The amp runs a pair of 6L6GC ( or EL34 ) tubes, it's cathode biased (25W 250R shared resistor) and is fitted with a Pentode/Triode mode switch. Whilst running on the GZ34 tube the B+ voltage was 420VDC. With the new rectifier installed B+ has risen to 450VDC. Plate voltage is 450VDC, Cathode voltage is 33.5VDC. According to my calculations that's a zero signal current of 67mA per tube. I know cathode biased tubes can run hotter than fixed bias but I'm wondering whether it's a bit much. Should I try to get B+ voltage back down or even put a different value cathode resistor in. Any suggestions?

[Michael Allen]

I would rebias first. How much current do you want? Adjust the cathode resistor accordingly

[pdf64]

'I have just replaced my GZ34 rectifier tube on my Epi Blues Custom 30 with a Sovtek solid state rectifier'
Why did you do this?
'Should I try to get B+ voltage back down or even put a different value cathode resistor in. Any suggestions?'
Put the GZ34 back in.
Sub in a WZ34 copper cap if you don't like the GZ34 for some reason.
Use a zener to reduce the B+.
But the GZ34 is best IMO, due to it's slow ramp up on the B+ at switch on.
Increasing the value of the cathode resistor will reduce the power output and likely cause excessive crossover distortion at high signal levels - you would likely find that regular grid bias would give a nicer overdriven tone, if you are keen to keep the B+ at 450V. Peter.

[ehedwr]

The reason I fitted a SS rectifier is because the GZ34 was arcing and blowing the main fuse on startup. I tried another and it was the same. It seems that filling the caps drags too much juice through the rectifier when I switch on the standby switch. Any harm in switching on the power with the standby switch on? I found that the GZ34 slowly warms up and allow current through more gradually. That's why I bought a bridge rectifier. As for the voltage on B+? If the figures I have given don't seem unhealthy then that's fine with me.

[olddawg]

Put two 10v 5watt zeners in the series on the B+ line if you want to keep the SS rectifier. They're cheap. Or use a copper cap. They're around $22.

[Prairie Dawg]

The reason I fitted a SS rectifier is because the GZ34 was arcing and blowing the main fuse on startup. I tried another and it was the same. It seems that filling the caps drags too much juice through the rectifier when I switch on the standby switch. Any harm in switching on the power with the standby switch on? I found that the GZ34 slowly warms up and allow current through more gradually. That's why I bought a bridge rectifier. As for the voltage on B+? If the figures I have given don't seem unhealthy then that's fine with me.

Leaving the standby switch on at all times seems to have been the solution to a lot of people with AC30 problems in toasting the rectifier tube. That's what I've done with mine.

[tubeswell]

If they are 6L6CG (max diss. 30W) then according to those voltages they are idling at 25.8W (Of the 67mA, I figured that about 5mA was screen current, making the plate current about 62mA per tube) which should be okay, since they are cathode-biased.

If they are EL34 (25W tube) you are at the limit, but if you like the sound then WTF - why not leave them as they are and see how long they last?

As an aside - the oft-quoted Merlin b has some tips for taming down the impact of start-up surges on tube rectifiers here. The Valve Wizard

(Although the bit about minimum limiting resistance is not really necessary for most amps - where the reservoir cap is low enough not to be a concern - this might not be the case with your amp - I couldn't find a schematic just now to have a look)

However as the others have said, one of them Weber copper cap rectifiers (WZ34) should handle it okay because they're not tube rectifiers