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112 RP 65 Not Loud

Working on a '79 112 RP 65, all parts date to late '78/early '79, came with EL-34s installed and in need of some TLC. It has the black background badge logos with silver lettering/characters.

Cleaned it up cosmetically and electronically, cleaned all pots, jack, sockets, fully recapped and biased it up correctly.

It is a very sweet sounding amp, great Phasor. Came with a clearly non-original reverb tank, replaced it with a recommended 4FB3A1B reverb tank that souded better, but still not particularly strong. Reverb is much better with a 8BB2A1B tank (like the one in my RD 50).

The big issue is that this amp doesn't get loud. My 210 65 (12AX7 PI) is considerably louder on Low power with Volume at 3 than the RP is at High power with Volume at 5 (Master Volume at 10 for both). Tried all combinations of Gain and Master Volume settings, in every case the RP 65 is significantly less loud. It will break up nicely with Gain at 8 and Master turned down, so I don't think the output transformer is bad. And voltage pretty much doubles at the power tubes when flipped from Low to High, so I think the power transformer is ok.

There was no tube chart in the cabinet, but it came with E34s and it has the large white 1.5K resistor on pin 4 of one of the power tubes. According to the schematics, EL34 is the right power tube for this amp. My understanding is that MM went to 6L6 power tubes in 1980.

Replaced all the opamps, didn't seem to make a difference

On a hunch, I installed a pair of 6L6 power tubes, rebiased, and both the volume and reverb improved dramatically. And the tone of the amp is as good as before.

Do I have some kind of transitional amp that requires 6L6 power tubes? Or are my driver transistors shot, and need to be replaced to sound right with EL34s? Would anyone have the proper voltages to check?

Suggestions welcome.


Should be EL-34

Only the GP-3A version of the RP amp uses 6L6, and yours is probably a GP-1. First off, the GP-1 came in a couple of different revisions and some of them are harder to drive. You can't compare a 2-channel amp's knob settings directly with an RP as the channel volume on the 2-channel is a simple voltage divider after a fixed gain stage whereas the RP has the 'gain' control as part of the feedback loop in that gain stage. The master volume works the same in both amps (identical if the 2-ch amp has solid state drivers).

Check the gain factor resistors for the first stage in the phase splitter in the RP (R48 and R49): If they are 10k and 18k, you only get a gain factor of 1.8 in that stage, an earlier version had 3.3k and 22k yielding 6.67 in amplification - quite the difference. For reference, a 65/130 amp has 4.7k and 15k for an amplification factor of 3.2 in that stage.

Something to start with anyways...

Lars Verholt


Will Check Gain Resistors and Swap Tubes

Thanks for the quick and detailed response Lars.

I will check the gain factor resistors to see what I've got onboard. Also thought to swap the EL34s the RP came with for the EL34s in my 210 65. Could get lucky and find the RPs tubes are just plain worn out...

If the tube swap yields no difference, and I have the lower value gain factor resistors (10k & 18k), is it safe to change them out for the higher value ones (3.3k and 22k)?

A buddy of mine is very interested in gigging with the RP if I can get it as loud as his HD-130 Reverb at half power. He tried the RP as is and said it just couldn't keep up at a jam.


Making the RP 65 Louder

Thinking about Lars's explanation of Gain Factor, I revisited the RP 65 schematics:

GP-1: 10K, 18K GF = 1.8
GP-2: 3.3K, 18K, GF = 5.45

Didn't see any RP version that had a 3.3K and 22K, so I decided to make my GP-1 like the GP-2. Tested this by paralleling a 4.7K resistor with the 10K resistor, yielding 3.2K. Sure enough, the amp was very noticeably louder. Tried progressively smaller parallel resistors, settling on a 3.3K yielding 2.48K and a GF of 7.25. Amp maintains its sweet tone, but with lots more volume at all settings. Still overdrives well too (Gain up, Master down).

Any danger is leaving this setup this way?


No danger

Hi there,

the 22k version is not documented in a printed schematic - it's in the notes section of the GP-1. There's no danger in the setup. You have essentially recreated close to the original 22k/3.3k (6 2/3) maybe a bit hotter still. Without knowing the actual reason, I think the 10k/18k was introduced in the RD-2 board to accommodate the extra gain stage, but the RP lacks.
The same board was supposed to work with both the 65 and 100 versions of the same amp.
The later 6L6 version of both RD and RP have different resistor values at this stage depending on whether it's 65 or 100 watts. I think it's perfectly safe to modify one of these quiet RP amps to have a bit more responsiveness.

I should note that the speaker in a particular amp is also a factor in the perceived loudness. I can highly recommend testing with the same speaker(s) when comparing amps - otherwise you are mostly comparing complete systems, which may be a valid comparison for other reasons, but for troubleshooting the amplifier itself you need a level playing field. Clear as mud?

Lars Verholt

PS - what kind of speaker does this RP amp have?


Final Thoughts - Making an RP65 Louder

I see the resistor changes in the Revision section, top right corner of the GP-1 schematic, thanks for pointing that out.

Going to check the bias again, recalibrate the Phasor, and call this one done. My friend will field test and let me know how he likes it now.

Norrmally I will compare two chassis through one speaker to keep the speaker from being a variable. In this case the volume difference was readily apparent both with the speaker the amp came to me with (older Pyle) and the replacement, an Eminence Legend. Also tried an Eminence Cannabis Rex (my favorite speaker for bright amp). None could compensate for the amp's innate low volume.

Can't thank you enough Lars for taking the time to help with this one, had me stumped for quite a while.


differences between GP-1 & GP-2

I discover that R48 / R49 difference and wonder if modifing my GP-1 is a good idea, as like it is now, it may be safer with volume raise when playing with pedals...

I also notice different C7 values, does anyone know what this C7 affect?


Personal Preference

If you like the way your amp sounds/performs now, why change it?

That said, you could easily clip in a resistor in parallel like I did to see if you like the increased output.


C7 interrogation

thanks for the reply Inertian;
your right, a R48 change would allow more volume, useful in some rare situations for me.
I'm considering a cap job for this 40year old good boy, it may be the opportunity for some upgrades.

So, with that "R48 volume improvement dilemma, looking at differences in schematics I noticed C7 different values and I wonder what it affects...



I am going to change C7 in my GP-1 unit to 0.1uF (GP-2 and later) as I find its bass response too boomy.

Lars Verholt



I imagined it affects bass response but wasn't sure...
please let know how the mod works.

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