Vox ac 30cc question

[chane]

Hi, I'm hoping someone can help me out with this question. I have a AC30cc with a blown gz34 and I am considering changing to a GTSS5AR4. My question is will I have to re-bias the power tubes?

thanks in advance

chane

[52 Bill]

Originally Posted by chane

Hi, I'm hoping someone can help me out with this question. I have a AC30cc with a blown gz34 and I am considering changing to a GTSS5AR4. My question is will I have to re-bias the power tubes?

thanks in advance

chane

In theory, yes because the solid state rectifiers will probably increase the B+ voltage. But seeing as this is a cathode biased amp with two different cathode resistor settings, you will need to change the resisitors to do this.

I don't remember how much voltage the B+ is normally in these, but someone here (Bob) should know if it is safe to up it a little with the SS rectifier.

[Robert M. Martinelli]

Hmm...well.....
The original AC30s were pretty rough on the power tubes, and these chinese versions follow the same guidelines....let us see....
+B can vary from 322 ( low filtering-vintege smoothing ) to 325VDC ( high filtering-modern smoothing ), let's consider the worst case ( bias switch set to "hot" and filtering/smoothing set to "modern" ). Our cathode resistor is 53 Ohms ( 150 Ohms paralleled with 82 Ohms ) and the resulting cathode bias voltage is 11 VDC - this means we have, say, some 314 VDC dropping across the power tubes, and the quiescent ( bias ) current is 11V/53Ohms=some 207 mAmps, roughly 52 mAmps per tube. Assuming each screen grid contributes with 8 mAmps, the quiescent plate current is 44 mAmps per tube, multiplying this by the voltage drop across the tube we get :
0,044 A *314 VDC =13,8 W ( quiescent plate dissipation ). That's hot IMHO.

Even if the cathode bias design is a self-adjusting design to some degree ( as the current increases so does the voltage drop across the cathode resistor and less voltage tends to drop across the tube ), I would not try to go higher than that ( with EL84s - 6BQ5s in ) - after all the maximum plate dissipation for EL84s is 12W according to the old databooks ( and we should bear in mind that the current production tubes are not on par with old Mullards or Philips miniwatts or Telefunken tubes ).

If you still want to try the "SS 5AR4" rectifier, be aware that doing so will shorten your output tubes' life, unless you switch to more rugged tubes like 7189s ( mil-spec version of the EL84 ) E84Ls ( special quality version of the EL84 ) or the russian 6P14P ( very nice and very cheap ). All these tubes have a maximum plate dissipation of about 14 W so they can probably withstand the extra stress imposed by the SS rectifier you're willing to use.

One last thing - if you like the way your amp "sags", just throw another 5AR4 in, as the SS rectifier will eliminate any "rectifier sag" due to its lesser differential resistance, so your amp will have slightly more headroom and less "sag" ( the "cathode sag" due to the cathode bias design will remain unchanged.

Hope this helps

Best regards

Bob

[chane]

thanks for the help confirming my suspicions and the information. i think i'll go ahead and give it whirl go round and i'll let you know how it works out. one other quick question. am i correct in assuming that i would be in essentially the same boat if i used one of the weber copper caps?

cheers

chane

[Robert M. Martinelli]

AFAIK the Weber "copper caps" are designed to emulate a tube rectifier while keeping the benefits of a SS rectifier ( they don't wear out ), so they're essentially a SS rectifier with a series resistor to increase the differential series resistance and approximate that of a tube rectifer. This means a Weber "copper cap" has a higher voltage drop and if compared to a SS 5AR4, it "sags" more ( more or less like a TR does ), while the SS 5AR4 does not "sag" and increases the +B ( because less voltage drops across it ).
Hope this helps
Best regards
Bob

[Bruce / Mission Amps]

Originally Posted by voxrules!

AFAIK the Weber "copper caps" are designed to emulate a tube rectifier while keeping the benefits of a SS rectifier ( they don't wear out ), so they're essentially a SS rectifier with a series resistor to increase the differential series resistance and approximate that of a tube rectifer. This means a Weber "copper cap" has a higher voltage drop and if compared to a SS 5AR4, it "sags" more ( more or less like a TR does ), while the SS 5AR4 does not "sag" and increases the +B ( because less voltage drops across it ).
Hope this helps
Best regards
Bob

But.... by comparison of the real GZ34 vacuum tube, the copper cap turns on within moments, while the real GZ34 takes about 15-25 seconds to start conducting.
In some amps with no standby switch that means all the tubes are hit with very high, unregulated DC voltage within moments... possibly well before the tubes have heated.
Is that an issue?
That's up to you.

[Robert M. Martinelli]

Hi Bruce,
thanks for pointing it out;
I didn't mention that issue simply because the amp we're talkin 'bout is an AC30CC and it does have a stand-by switch so the tubes are not going to be "hit" turning the amp on, even with a SS rectifier, provided that we give 'em some time to warm up.....I was analyzing the "sag" issue because sound-wise it makes some difference on dynamics, and the +B voltage increase issue because AC30s are pretty rough already on output tubes, so I wanted him to be aware of that.
Best regards
Bob

[Rick Erickson]

My vote would be to put a decent quality 5AR4 in there and go play. Alternatively, a couple 1N4007's and a 100 Ohm 10W or better series resistor would probably do the same thing as any SS plug-in rectifier. Bruce is correct about the Copper-Cap. The claim is that they turn on slowly, but the reality is that hit the tubes immediately. This won't be an issue IF the Standby switch is used properly.

RE