HRD ML212 "Clone" Build Reverb Issue

[W5FH]

Hello-
   I am new to this group and this is my first post. My son in law is a performing blues guitarist (Chris Cameron of Arkansas) and I built him a tube amp based on the Fender Hot Rod Deville ML212 design. I realize now that I made an error in adapting a tube reverb circuit to it and wish for some advice from the folks here.
    My background is Ham radio, I have held a license since 1967. I have built a lot of radio equipment both tube and solid state over the years, having a large collection of electronic parts and a workshop for building. Chris wanted a very clean amplifier (pedal platform) and told me the Fender HRD ML212 was the closest to his desires that he had played through. Parts references here are per HRD ML212 schematic. So I duplicated the 3 triode gain stages (V1-A&B, V2-A) leaving out all the switchable effects (bright, boost), effects loop and solid state reverb. I did include components that would be in the circuit at all times associated with these 3 triode stages. The third stage has no cathode bypass capacitor. One thing I tried as an experiment is that I used a 50 uFD film capacitor for the first stage cathode bypass in lieu of the 47 uFd electrolytic. I also duplicated the phase inverter and power amplifier of HRD ML212. I used the Hammond 290DX power transformer, #194B choke and Heyboer output #8025. The power supply, filament wiring and grounding was all done per tutorials found on Merlin Blencowe's site and preamp book. I used selected surplus US made 12AX7's in preamp/PI and a pair of NOS USA 1956 dated TungSol 5881's in power amp. One thing I did was to use a large amount of capacitance in the HV power supply, after the choke for power amp screens and throughout the series dropping resistors in preamp B+ string.
    I used the old standard Fender tube reverb circuit of the paralleled 12AT7 triodes driving an output transformer to pan (Accutronics #4AB3C1B) and a 12AX7 triode recovery stage. On the Fender HRD ML212 schematic they show a 9.15 mV 1 Khz sine wave input to the high impedance input jack. R-40 in the HRD ML212 is the reverb mix resistor. This 220K resistor is taking the 1.45 VAC signal from output of third triode gain stage (V2-A) and coupling it to the phase inverter. The HRD ML212 shows a 920 mV AC signal on downstream side of R-40. I placed the tube reverb circuit across this same resistor. The output of the reverb recovery triode stage is in the 100 mV AC range so the reverb in this amplifier is very subtle. I now realize my mistake.
    The amplifier was built on a Seaside Music chassis and incorporated into a Guitar Center Deluxe Reverb cabinet with 12" Celestion speaker. The amp has worked out beautifully for Chris, he tells me that it has the best tone of any amplifier he has ever played through. So far it has been used in about 30 gigs. Chris uses pedals for most effects, he does not play the amp real loud as most gigs are fairly small however it has made a few 400+ folk gigs and did well at higher volumes. Chris is OK with the reverb as it is and he really does not want me to disturb the present tone when I discuss fixing the reverb.
    My first though is to continue to use R-40 as reverb mix resistor and just add another triode gain stage following the single triode reverb recovery stage. This will get the reverb wet signal back into the 920 mV level on downstream side of mix resistor R-40. I don't want to use a high value reverb mix resistor as was done in the early Fender designs; I believe that would hurt the tone (and gain distribution) of this design. I have been researching reverb schemes and I do see some that are avoiding using the high value (2 to 3.3 Meg) mix resistors.
    I would appreciate any help. I am preparing for two more builds of this same amplifier design, having all parts here now.
Byron Tatum

[sluckey]

It would be very helpful if you post a schematic of the amp as it is right now.

[W5FH]

Thank you for the reply -
   I apologize for the hand drawn schematic. It is showing the three preamp stages and R-40 reverb mix resistor. I duplicated the old Fender tube reverb circuit that uses a paralleled 12AT7A driving the pan through an output transformer with a single triode recovery amplifier (12AX7A one triode). This is the one as used in AB763 design, etc. I am a little hesitant to disturb anything in the preamp and at this time am considering adding another reverb recovery gain stage in order to better match the 920 mV AC signal on downstream side of R-40.

[sluckey]

I would increase the size of R40 to knock the dry signal down. This effectively increases the reverb signal relative to the dry signal. Then put another triode between R40 and the PI input to bring everything back up to drive the PI. The overall result should give a much stronger reverb.

Look at the Custom Vibrolux schematic. R32 is the equivalent to your R40.

     https://el34world.com/charts/Schematics/files/Fender/Fender_custom_vibrolux_manual.pdf

[W5FH]

Thank you very much for this help and the example schematic to study (Fender Vibrolux Reverb).
        If you have another moment here, I would like to ask you about something I noticed on the Vibrolux Reverb schematic. It involves the coupling capacitor C-18 and resistor R-36 that feed the signal from last triode gain stage V4-B to the phase inverter input. C-18 is only 500 pF and R-36 is 1.0 Meg. From what little bit I understand (at this point in my learning) it seems to me C-18 would overly restrict lower frequencies. The HRD ML212 has a 0.022 uFd capacitor in that spot and much less series resistance. Can you tell me the reasoning for selecting C-18 and R-36 values in the Vibrolux Reverb? Thank you sluckey!

[PRR]

... Vibrolux Reverb schematic. It involves the coupling capacitor C-18 and resistor R-36 that feed the signal from last triode gain stage V4-B to the phase inverter input. C-18 is only 500 pF and R-36 is 1.0 Meg. From what little bit I understand (at this point in my learning) it seems to me C-18 would overly restrict lower frequencies. ...

Good eye. Couple of things:

Normally we say C18 "feeds" R38. That would be 318Hz.

But R36 is in the same circuit. So the gain is half (it seems) and the bass-cut moves to 154Hz.

BUT. The other end of R39 goes to the cathodes of the LTP. Not a ground. It moves with the signal. "How much" depends on several details. But something like half. This "bootstraps" R39 to about twice the value. 77Hz. Essentially the low-note on guitar. (It's not a brick-wall filter.) It also is slow to grid-block and quick to recover.

[W5FH]

Thank you very much PRR -
     You have opened my eyes some more in my learning. I have been reading the book by Mr. Blencowe on tube guitar preamplifier theory. Right now into the chapter on coupling. I see what you are saying regarding the bass cut-off frequencies and dealing with blocking distortion in regards to the component values.
     The Custom Vibrolux Reverb has 3 triode gain stages with the third one identical in design (100K plate load/1.5K cathode/Unbypassed) to same third triode gain stage in the Fender HRD ML212. However, in path from plate of this third gain stage to phase inverter input (and phase inverter circuitry) these 2 amplifiers differ in design. The CVR has no negative feedback whereas the HRD ML212 has negative feedback/presence control. Since I have not yet studied in depth the theory of the phase inverter and negative feedback I do not want to indulge into your time too much here but wanted to ask for your commentary on the reasoning of the two amplifiers regarding component selection between this identical third gain stage and phase inverter.
     Again, a big thank you to you guys for your help and time here.   

[thetragichero]

keep in mind the 500pf coupling cap into the phase inverter was pretty standard fender fare in the blackface era (and in some models beyond), i think even for the bassman