Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Pochie45566 on February 21, 2018, 08:06:56 pm

Title: swapping 5u4gb to gz34
Post by: Pochie45566 on February 21, 2018, 08:06:56 pm

I have a 2204 which the power transformer died in the past and someone swapped in a Sunn 200s PT into which has a much higher b+ voltage so to compensate for this, they threw in a 5u4gb rectifier for the solid state one. That just makes me sick  :laugh:. But if I convert it to solid state, my already 500v b+ will jump to like 550 and that'll dramatically impact tone and shorten the life of my el34's. If I swap the 5u4gb for a gz34 to keep a remotely marshall tone how much will my b+ raise and how can i compensate for it. Ideally I convert it to a solid state rectifier but I don't think that would work. Any advice is great! thanks.

Title: Re: swapping 5u4gb to gz34
Post by: VMS on February 22, 2018, 04:08:08 am

Easiest thing to do is to sell that PT and buy proper PT for your amp.

Title: Re: swapping 5u4gb to gz34
Post by: tubenit on February 22, 2018, 07:30:53 am

500 divided by 1.2 (5U4gt) = 416 v X 1.3 (GZ34) = 541 v.

The schematic for a 2204 normally shows 400v on the power tube plates.     

http://el34world.com/charts/Schematics/files/marshall/Marshall_2204%20JMP%20Master%20Volume%20Lead%2050watt.gif

If you got a 300-0-300 PT with a GZ34, you'd have 390v and with a solid state plug in rectifier like Hoffman sells you'd have 420v.  (guesstimates)

With respect, Tubenit

Title: Re: swapping 5u4gb to gz34
Post by: mresistor on February 22, 2018, 10:12:17 am

Classic Tone has one that has selectable HV secondaries http://www.classictone.net/40-18095.html

Title: Re: swapping 5u4gb to gz34
Post by: 92Volts on February 22, 2018, 03:00:20 pm

Some combination of zener diodes and resistors can reduce voltage with different amounts of "sag"... dropping lots of voltage with little sag becomes a pain in the ass. Zener diodes drop consistent voltage with varying amounts of current, but each diode can't take much power so you end up needing lots of them.

There have been Marshalls in the past with tube rectos, so I'm not sure that's incompatible with "Marshall tone". This particular amp wasn't designed that way, of course. But is there a problem with this amp's tone you are sure can be attributed to the tube recto? Maybe there is one or more other problems or ill-advised modifications that have happened to this specimen...

Title: Re: swapping 5u4gb to gz34
Post by: PRR on February 22, 2018, 08:19:49 pm

Put in KT88s and use the extra power (70W-80W).

Title: Re: swapping 5u4gb to gz34
Post by: mresistor on February 23, 2018, 02:42:32 pm

Quote from: PRR on February 22, 2018, 08:19:49 pm

Put in KT88s and use the extra power (70W-80W).

Or KT77's.