Morgan RCA35 - Not What I Thought

[Bwmitc9285]

Hey All! Had a friend drop off his Morgan RCA35 with me last week. The problem he was having was that the 6v6 tubes he had in there were red plating not too long after turning on. Morgans website says its safe to run 6v6 tubes in this amp but seeing as its cathode biased and is advertised as running at 35 watts with 6l6 tubes I was immediately concerned this probably wasn't a good idea. The power tubes in this amp share a 250ohm 10w resistor and I was getting a 35 volt drop across this which is 180% dissipation per tube. Put a set of 6l6 tubes in and all is well again.

However what really surprised me though is the preamp. This amp is advertised as having "big bottle" American amp tone but after tracing the preamp I realized its basically a Superlite TMB preamp. It has a 220k plate load resistor instead of 100k on V1 and some capacitor value changes throughout but overall is really similar. The amp sounds really good but based on the advertising verbiage I thought this was going to be a AB763 variant into a big bottle vox-ish power section. Turns out its not really an American style amp at all. Attached a rough schematic up until the power section in case any one is curious.

I thought this was really interesting and thought you might all think so too. Good lesson to not judge an amp by its cover.

[dwinstonwood]

I don't think I've ever seen a 1M Mid pot in another amp.

[Bwmitc9285]

I've seen a mod for tweed deluxes that uses a 1m for a mid control but not in typical TMB tone stack like this. The description says the mid control is designed to add gain so I am wondering if the intent of the 1M is to basically lift the tone stack out of the circuit at high settings.

[dwinstonwood]

I did a similar thing on an amp by using a switch to break the ground connection, and it did add quite a bit of gain.

[shooter]

shouldn't the volume pot wanna stay grounded, whether the TS is floating or not?

[dwinstonwood]

shouldn't the volume pot wanna stay grounded, whether the TS is floating or not?

Yes! It was in the amp, just not in my drawing. 
[I fixed it]

[pdf64]

I've seen a mod for tweed deluxes that uses a 1m for a mid control but not in typical TMB tone stack like this. The description says the mid control is designed to add gain so I am wondering if the intent of the 1M is to basically lift the tone stack out of the circuit at high settings.

Surely the mid pot was wired as a variable resistor, rather than a potential divider as shown?
Because as per the latter, the bass control would have very little effect, the amp would always be super bassy.

[HotBluePlates]

... Morgans website says its safe to run 6v6 tubes in this amp but seeing as its cathode biased and is advertised as running at 35 watts with 6l6 tubes I was immediately concerned this probably wasn't a good idea. The power tubes in this amp share a 250ohm 10w resistor and I was getting a 35 volt drop across this which is 180% dissipation per tube. Put a set of 6l6 tubes in and all is well again. ...

What matters is the screen voltage in that amp.

Assuming reasonable voltage, you will find that cathode-biased 6V6 amps use a shared 250Ω cathode resistor, and cathode-biased 6L6 amps use a shared 250Ω cathode resistor.

It only gets out of whack when the screen voltage gets too high (though that also implies higher plate voltage), causing the 6V6s to over-dissipate at idle.

[Bwmitc9285]

I've seen a mod for tweed deluxes that uses a 1m for a mid control but not in typical TMB tone stack like this. The description says the mid control is designed to add gain so I am wondering if the intent of the 1M is to basically lift the tone stack out of the circuit at high settings.

Surely the mid pot was wired as a variable resistor, rather than a potential divider as shown?
Because as per the latter, the bass control would have very little effect, the amp would always be super bassy.

Yeah the amp is incredibly bassy. Even with the bass on 0 it’s still very boomy even with a tele. Definitely not an amp for humbuckers imo.