Carvin Legacy Pentode to Triode Mod

[Ruwaldo]

Hello All,

I have a Carvin Legacy 2x12, 100 watt combo that I'm interested in trying to mod to run the power tubes in triode configuration. My main problem is that the amp is just waaaaaay too loud even when only using two tubes (50 watt mode). In my looking at different options I came across a site that talked about modding the output stage of an amp to run the power tubes in triode which will change the sound of the amp and also make a large reduction in output power.

http://www.people.cornell.edu/pages/...converter.html

I understand the difference between a pentode and triode tube but my concern is that the site makes it seem too easy, just slap a resistor betwen pins 3 and 4! While I want to try this mod I also want to do it properly and not do any damage to my amp.

If any of you great tube guys would be willing to take the time to look at the schematic for my amp and give me your input on this mod I would really appreciate it. I do have a background in electronics and have done some biasing and minor repairs on tube amps in the past but I need some help doing this mod right.

Here is the power section of the amp:



Thanks in advance for your time and advice. It's great that so many of you are willing to share what you have spent so much time learning.

Later,

Russ

[Enzo]

No need to make it more complex than it is. The resistor limits screen grid current, as opposed to just shorting it to plate. His adaptors ought to work.

Are you in Australia? I notice the schematic is upside down.

One thing about loudness, pulling half the tubes drops the power by half, but that is only a 3db drop. To get to half as loud, a 100 watt amp needs to become a 10 watt amp. To be 1/4 as loud as the 100 watt amp, you need a 1 watt amp.

Sometimes it is easier to rotate the volume knob.

[Ruwaldo]

Thanks for your reply Enzo,

Strange about the schematic being upside down when you view it. It's fine whan I look at it.

This amp doesn't have a master volume control and since the level sent out the effects loop is controlled by the channel volume setting it's pretty much impossible to get a usable level out to my TC Electronic G Major when I have the amp turned down to a useable level at home.

It was my understanding that running the amp on two power tubes in triode operation would drop the output to around 20 watts. This should get me into a usable power range.

As far as making the adapters as he showed on his site I just don't have the room in the amp to mount them. The way the cab is built there is only about an inch between the bottom of the power tube and the wall of the cab. This is another one of hte reasons why I didn't consider something like the THD Yellow Jackets. Because of this I'm going to actaully make the mod to the circuit inside the amp. I just need to know if it's actually as simple as just putting the resistor from pins 3 to 4 on the output tubes.

Thanks,

Russ

[hasserl]

Russ, the schematic appears right side up to me. Though it is too small to see well. I've got a copy of it though.

The two wires to the output transformer center tap and to the screens are carried together in a cablebetween the large and small pcb's, the connector is labeled H3 on the pcb's. IIRC it is a two wire cable, "a" goes to the OT center tap, "b" goes to the screens. You could tap into that cable, running it to a toggle switch that would allow you to switch the "b" wire for the screens to "a" wire, so that both the screens and the plates are served from the same source, this will give you the triode mode. Being on a toggle would allow you to switch it back to standard pentode operation if you wanted. However, Carvin runs only 350 ohm screen grid resistors on the pcb. I would upgrade these to 1k resistors to offer better protection for the screens.

Also, I expect there would ba a change in the tone with triode operation. That has been my expereince. Whether you'll like it or not is subjective, but the tone tends to be darker with less distortion. And I think triode operation may be harder on the tubes anyway, I think the screens will carry much more current in triode operation, which is another reason I would upgrade those screen grid resistors to 1k.

[Enzo]

Still wrong way up on my screen...

I wasn't suggesting to make his adaptors, just that they ought to work, so duplicating his circuit in your amp ought to as well.

[Ruwaldo]

Thanks again for your input guys.

I think I'll change out the resistors to the 1Ks suggested and then make a Y cable to adapt the screens to the center tap supply. This will be an easy way for me to actually see and hear what the effect of the mod will be.

If it sounds good then I'll look at working in a switch to enable both modes of opeation. Since this switch will be used to change the high voltage supply to the tubes would it be a good idea to have a relay circuit make the connections to isolate the switch, and the person in contact with it, from the high voltage levels?

Thanks again for the help,

Russ

[Enzo]

No more so than the standby switch which usually has hundreds of volts in it.

If you used relays, then you'd need big relays that are rated for B+ voltage. A plain old DPDT toggle ought to work.