Rectifier Tube question

gtrplr2 · April 12th, 2005, 09:21 PM

After a blackface mod has been done on a 1972 Super Reverb, should or could, the 5U4G rectifier be changed to a GZ34? What effect would this have on the amp? Thanks,

Jeff

5965 · April 13th, 2005, 11:25 AM

In general, the GZ34 tends to tighten the sound up and make it a bit more focused, while the 5U4GB is softer and more loose. The tone itself doesn't change much, just how it's presented and how the amp feels.

For most applications and usages, they're fairly interchangeable. However, I've heard disagreement on whether there may be problems with voltages and certain transformers in specific circumstances.

I'm not really familiar with your amp and its parts, so I can't give you an opinion, let alone an answer. For home use, it probably wouldn't matter under any circumstances. If there were going to be issues, high volume gigging would be where they'd probably show up.

Even there, however, disagreement would exist, and it'd be more over whether the imbalances actually caused harm than whether they conceivably might. Is that confusing enough?

Tremo · April 13th, 2005, 03:36 PM

The PT is designed to use a 5U4, and no amount of BF modding is going to change the turns ratio of the PT, short of replacing the PT.

Here's what I'd do. First off, with a 5U4 in there and the amp properly biased and idling (no signal), measure the actual plate voltages on the 6L6s. If the actual voltage is 440 or higher, leave the 5U4 in there.

If the voltage with the 5U4 is under 440, then swap in a 5AR4 and rebias. Measure the plate voltage again. Anything under 470 or so will be OK.

The higher the 6L6 plate voltage, the more power and headroom the amp will have.

gtrplr2 · April 13th, 2005, 03:46 PM

Quote:

Originally Posted by Tremo

If the voltage with the 5U4 is under 440, then swap in a 5AR4 and rebias. Measure the plate voltage again. Anything under 470 or so will be OK.

The higher the 6L6 plate voltage, the more power and headroom the amp will have.

Thank you for the reply to my question. I will measure the plate voltage and see what it is, as you suggested.

gtrplr2 · April 13th, 2005, 03:51 PM

Quote:

Originally Posted by 5965

Even there, however, disagreement would exist, and it'd be more over whether the imbalances actually caused harm than whether they conceivably might. Is that confusing enough?

Thanks for the reply. It is a bit confusing. I have looked at the interchange chart for rectifier tubes on the KCA NOS web site. It looks like it is one that could be changed. It does not look like it should be a problem with careful monitoring at first.

Rick S · April 13th, 2005, 04:24 PM

Quote:

Originally Posted by gtrplr2

Quote:

Originally Posted by 5965

Even there, however, disagreement would exist, and it'd be more over whether the imbalances actually caused harm than whether they conceivably might. Is that confusing enough?

Thanks for the reply. It is a bit confusing. I have looked at the interchange chart for rectifier tubes on the KCA NOS web site. It looks like it is one that could be changed. It does not look like it should be a problem with careful monitoring at first.

Yes, confusing to say the least. Read Tremo's post and follow his advice to the T. As usual, his information is spot on regarding this.

I think the bottom line on this is that people seem to think that using the GZ34 in a 5U4 amp would seem to be a logical step towards blackface performance, and that is simply untrue.

In the amps with the 5U4 transformers, your voltages will be closer to blackface specs with the 5U4 in place. I always leave the 5U4 in these amps.

lastima · April 14th, 2005, 11:43 AM

Not confusing. The guy knows what replies are coming, doesnt agree with them and doesnt want to get involved.

Lots of guys like 5U4's in the bfsr better anyhow.

fezz parka · April 14th, 2005, 04:32 PM

Quote:

Originally Posted by lastima

Lots of guys like 5U4's in the bfsr better anyhow.

FWIW, I wouldn't run a 5u4 in a BFSR. The heater current draw would be right on the edge. A Wu4 copper cap, yes. A real 5u4, no.:D

Tremo · April 14th, 2005, 06:19 PM

Quote:

Originally Posted by fezz parka

FWIW, I wouldn't run a 5u4 in a BFSR. The heater current draw would be right on the edge. A Wu4 copper cap, yes. A real 5u4, no.:D

The original poster has a 70s SFSR, that has been BF modded. His PT is set up for a 5U4, so no problems there.

But with a real BFSR, I agree. 5AR4 or Weber copper cap.

April 12th, 2005, 09:21 PM	#1 (permalink)
gtrplr2 Tele-Meister Join Date: Apr 2003 Posts: 198	Rectifier Tube question After a blackface mod has been done on a 1972 Super Reverb, should or could, the 5U4G rectifier be changed to a GZ34? What effect would this have on the amp? Thanks, Jeff

April 13th, 2005, 11:25 AM	#2 (permalink)
5965 Tele-Holic Join Date: Apr 2003 Posts: 738	In general, the GZ34 tends to tighten the sound up and make it a bit more focused, while the 5U4GB is softer and more loose. The tone itself doesn't change much, just how it's presented and how the amp feels. For most applications and usages, they're fairly interchangeable. However, I've heard disagreement on whether there may be problems with voltages and certain transformers in specific circumstances. I'm not really familiar with your amp and its parts, so I can't give you an opinion, let alone an answer. For home use, it probably wouldn't matter under any circumstances. If there were going to be issues, high volume gigging would be where they'd probably show up. Even there, however, disagreement would exist, and it'd be more over whether the imbalances actually caused harm than whether they conceivably might. Is that confusing enough?

April 13th, 2005, 03:36 PM	#3 (permalink)
Tremo Banned Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,803	The PT is designed to use a 5U4, and no amount of BF modding is going to change the turns ratio of the PT, short of replacing the PT. Here's what I'd do. First off, with a 5U4 in there and the amp properly biased and idling (no signal), measure the actual plate voltages on the 6L6s. If the actual voltage is 440 or higher, leave the 5U4 in there. If the voltage with the 5U4 is under 440, then swap in a 5AR4 and rebias. Measure the plate voltage again. Anything under 470 or so will be OK. The higher the 6L6 plate voltage, the more power and headroom the amp will have.

April 14th, 2005, 11:43 AM	#7 (permalink)
lastima TDPRI Member Join Date: Jan 2005 Posts: 18	Not confusing. The guy knows what replies are coming, doesnt agree with them and doesnt want to get involved. Lots of guys like 5U4's in the bfsr better anyhow.