Pulling tubes? Peavey Rock Master head

[f13ndus]

Hey all, got a quick question - don't know if anyone here knows the amp, but I just picked up a Peavey "Rock Master" head in a trade. Not to be confused with the newer Rock Master preamp, this is a stand-alone head with 4 x 6L6GC power tubes and 3 x 7025/12AX7 preamp tubes. 120W output at 4ohms, according to Peavey.

I'm wondering if I can pull the two outside power tubes to run at half power? I know the volume won't drop much, but I'd like a bit more "fullness" out of the amp. The amp wants to see a 4ohm load, so with two tubes pulled I should be using an 8ohm load, correct?

The output transformer in this amp is huge (in my opinion, of course, this is my first 100+W amp) and I know older Peavey gear was built like tanks, so I'm not too concerned about stressing it. I'm sure Peavey designed this thing to still work if, say, plugged into an 8ohm cab instead of 4ohm. I'm just not sure if there's anything funny about the power section that would prevent me from pulling tubes - I assume it's running a parallel push-pull output like most quad 6L6/EL34 amps, but I don't want to pull tubes until I'm sure.

If anyone has a schematic for this amp, I'd appreciate it - I plan to tweak it in the future, but for now I just want to get used to it as-is.

Thanks if you can offer tips or advice!

Attached a picture - yes, I've got the cabinet as well, just pulled the chassis to clean it - tubes look like all-original Peaveys, and the dust in there made me think it's never been opened before!

[f13ndus]

As a side note, I just looked at the power tubes more closely - they're made in the USA and, although Peavey labelled, are also marked STR-387 - looks like they're Sylvania/Phillips tubes, supposed to sound great, and if they test out good, probably worth quite a bit more than what I traded for the amp - good deal!

[Enzo]

Get the schematic from PV customer service. Schematic heaven might have it too.

YOu have the impedance backwards. If you pull two tubes, you need to plug your 8 ohm speaker into the 4 ohm jack. Unless you have a 2 ohm jack, don;t use the 4 ohm cab.

These are loud as hell, but there is nothing unusual about them. I have one of my owm out in the warehouse.

[Mark Black]

Enzo probably forgot he already posted this schemo. I sent a scan of my own paper copy to Schematic Heaven a long time ago as well but they never posted it.

Here's Enzo's version (looks better than mine and has the layout).

I've got one of those beasties in my stash as well - used it as my stage amp for quite a few years but it developed problems and became yet another back-burner project...

[Enzo]

That came from the factory.

My Rock Master needs me to open it up and make the heater supply connections whole. One of those ribbons or something is intermittant.

[f13ndus]

Originally Posted by Enzo

Get the schematic from PV customer service. Schematic heaven might have it too.

YOu have the impedance backwards. If you pull two tubes, you need to plug your 8 ohm speaker into the 4 ohm jack. Unless you have a 2 ohm jack, don;t use the 4 ohm cab.

These are loud as hell, but there is nothing unusual about them. I have one of my owm out in the warehouse.

Enzo, I don't think I got it backwards - I said "The amp wants to see a 4ohm load, so with two tubes pulled I should be using an 8ohm load". The amp with all the tubes wants to see 4 ohms - doesn't have an 8ohm tap on it. So if I pull two tubes, it'd want to see an 8 ohm load on that "4 ohm" tap.

Just to check that I understand the theory behind this - the output transformer matches impedance, in this case the 4ohm impedance of a speaker cabinet to the much higher output impedance of the power tubes. (not sure what the actual impedance of a quad of 6L6s would be... I should know that by now) Since that quad is actually two pairs of parallel tubes in push-pull, and parallel wired tubes have half the impedance, when you pull two tubes to lower power, the output impedance is doubled, so the load that should be on the other side of the OT should be doubled. Is that about right?

Good to know that I can pull tubes, though! I only asked because somewhere in my mind I vaguely remember someone somewhere saying that some amps have something in the power section that would fry the amp if you pulled tubes (yep, can't get much vaguer than that... )

Mark Black, thanks for posting that schematic and board layout! It'll come in handy down the line.

[Enzo]

You are quite right, I didn't read your post correctly. Use an 8 ohm cab in the 4 ohm jack, yes.


What I thought you wrote - and I see you did not - was wanting to plug the 4 ohm load into an 8 ohm jack.

I never really thought about it before, the amp has only 4 ohm and 2 ohm outputs.