The Amp Garage

Could I swap the wires on the Out Jack instead

No. Just swap the red and white primary wires. They are plenty long. Just route them under the board.

Here's the popular one wire mod. Maybe this is what you have...

Set your meter to read AC voltage, not DC.

The green wires from the PT connect to a terminal strip. That's a convenient place to pick up 6.3VAC.

When you remove a pair of parallel tubes on the primary, the impedance increases. Therefore you need to increase the impedance on the secondary as well. Just think of the parallel tubes as two parallel resistors. If you remove one of the parallel resistors the total resistance increases. If the resi...

xtian wrote:Danger! If your amp normally wants to push against 16 ohms (16 ohm tap) but you remove two power tubes, you need to reduce the load by half to 8 ohms. Sounds like you've got a 1:4 mismatch which is a recipe for disaster.

I think you have that turned around?

is the center tap(tan, red) on the chart the only ground on the o t?

That tan/red wire will connect to B+, not ground.

Stevem wrote:The 5f4 schematic I just looked at

Can you post that schematic?

I've been using this "potato masher" 5R4 with 6550s for years in my Sunn Sceptre.

[img500]http://home.comcast.net/~seluckey/amps/sunn/sunn_02.jpg[/img]

V1, V2, V4, and V5 ALL have zero volts on the cathodes. That means none of those tubes are passing any current. Look for a common problem, such as no ground connection for those tubes. Measure resistance from those cathodes to ground. One probe directly on the socket pin, the other on chassis. Proba...

What if you just wanted a Single In for the Power Amp.? Then don't install the preamp out jack. You don't need a two circuit TRS jack. Just a simple switchcraft 12A (single circuit with switch) will do. The preamp will be disconnected from the PI when you insert a plug into the power amp input jack...

I was going to say it looks like a universal PP tranny, with the #4 connection ("B") as the center.

Ah, that makes sense.

It appears to be a single ended OT. AC-15 uses a push/pull output.

This is how I do it. Also works as a passive effects loop. Just be sure to put the jacks before the input cap for the PI.

http://home.comcast.net/~seluckey/amps/ ... vember.pdf

Have you actually built this? No, but some guys over on Hoffman's board have. I have this layout from some where and i have wondered if there would be much of a volume jump when switching from the 1987 to the JCM channels? I have wondered if there needs to be a volume control for the JCM stage? The...