Peavey Road Master Hack

[silverfox]

What sort of complications would I run into trying to take the preamp board from this Peavey Road Master, then build a new supply and a Fender 6L6GC based power amp of about 40 watts.

This amp is entirely too big, heavy, and loud for my purposes. It does have a really nice sound and I thought downsizing it would be a good alternative project. I've got Iron from a 40 watt amp.

How would I determine the preamp to power amp matching? Challenges, Problems?

Schematic: http://www.el34world.com/charts/Schematics/files/peavey/peavey_roadmaster.pdf

Fender Pro Reverb: http://www.el34world.com/charts/Schematics/files/fender/PRO_AMP_AB763.pdf

Thanks,

Silverfox.

[sluckey]

You'll need a low voltage supply for the LDR switches. Other than that, just use the preamp output. Connect it to the input cap on the PI for the AB763.

[PRR]

> to the input cap on the PI

Heck, why not keep V5 V6? This driver "is" the classic Fender long-tail except beefed-up to smack a 3-pair team. It can just as easily drive 1 pair. This retains the Presence control which is _not_ on ProAmp AB763, but could-have-been, or you can bust-off that pot (replace with 10K fixed, or use the 820:47 divider of AB763). A 1-pair team can be hand-wired quicker than modifying P-V's 6-pack PCB.

The +25V needs 25mA. (Actually you want a steady 15V with up-to 25mA at point 'A'.) If you run a Fender-type bias-tap at 50VAC, add the other diode and a 100uFd 100V cap to get +70VDC, to a new R6 which is 2,200 Ohms at 5W. Take the +15V at point 'A' to the heater bias. (Tho personally I'd cut V1 V2 V3 heaters out and give them pure DC; the other heaters can take any old power.)

[silverfox]

Thanks all.

I had considered just dropping in lighter Iron and removing 4 tubes. One of the windings on the proposed PT is a bias winding that powered EL34's and originally was used for 7686 tubes so that may well work for the low voltage DC source too. I'll have to check the filament current ratings to see if it will support the tube count.

How much leeway is there on the Plate and Screen voltages? If the voltages dropped by 20 volts but kept the same relationship I figured that should still work?



In other news- The Univox 1221 the Forum assisted me with turned out great. I still haven't swapped 6550's in yet but I use it as is. The preamp circuit caps may have self healed after being used for a while since it quit the occasional popping every now and then.

Thanks for the suggestions,

Silverfox.

[PRR]

> If the voltages dropped...

There's no absolute-value voltages in there (such as Silicon clipping-diodes). If everything drops 20% there's no real change, allowing for the new goal being 40W instead of 160W. "Feel" should not change, or not near as much as the obvious power (and weight).

[silverfox]

http://www.el34world.com/charts/Schematics/files/peavey/peavey_roadmaster.pdf

To recap, I want to down size the amp by replacing the trannies with lower power units and rebuild the output section (as suggested by PRR).

About ready to start this project and at this point I'm considering the purchase of a PT and OT as opposed to using something on hand. Probably a 40 watt Fender power transformer would work; It appears with two 6L6GC output tubes; 6 preamp tubes = 4 amps in 6.3v, (overkill?); 320 0 320, (borderline?), 140ma HT, (borderline); and a bias tap at 50v; I'd prefer to find one with a 25 volt tap but that would likely be too expensive and unnecessary in light of the suggestions.

What are some opinions regarding the tone characteristics of various output transformers in the 40 watt range, e.g. Vox vs Marshall vs Fender.

I'd prefer to purchase from Doug.

A good overview on OT and the influence on tone if you've got an opinion on that. I understand this would be a highly subjective topic.

Thanks for the considerations,

Silverfox.

[silverfox]

I've started this project now.


PRR said: "The +25V needs 25mA. (Actually you want a steady 15V with up-to 25mA at point 'A'.)"

I have a 15 volt regulator chip that came out of an Organ circuit. Plan to use that after dropping the Bias voltage below 35 volts. But, don't I need 24 volts on the Status Circuit? I could do without the Status LED since it is only a light.

The donor Iron came out of the Univox: http://www.el34world.com/charts/Schematics/files/univox/univox1221.pdf

Peavey Road Master: http://www.el34world.com/charts/Schematics/files/peavey/peavey_roadmaster.pdf

I'm not sure that will provide enough filament current for all the tubes. Is it 350 ma/ filament with a total of 700 ma per 12A_7 or is the 350 ma for each tube total. I know the 6L6's require 900 ma/ tube.


Silverfox.

[silverfox]

I'm calling this done for the following reasons:

I've gotten the amp to work. However The preamp portions sound crappy. Tinny and weak.

The Univox power transformer didn't work, I suspect, since it was designed to power a voltage doubler circuit and I couldn't get the supply voltage up beyond 350 volts; insufficient current? (btw, some would question stripping a power tranny from a working amp.. Not the case. The Univox needs a recap and I bought it specifically for the transformers)

I'm using what amounts to a Fender 40 watt power transformer for the power supply and the Univox 100 watt output transformer. That tranny was designed for 6L6 power tubes.

The power amp sounds fantastic. However, there is either a speaker excursion, (unlikely as these are 120 watt speakers from a B-52 cabinet), or a design anomaly  or signal distortion; But, this only appears at higher volumes. I still need to check the bias and I'm going to change the supply to a choke design. The Presence control must be kept around 9 or a high pitched squeal develops. Other wise it sounds fantastic with an effects box acting as a preamp.

As it stands I would like to reopen this project as a new Topic with the following design goals- Get the power amp debugged and then come up with some preamp designs to build, either by changing the resistor values so the tubes function properly or modifying the existing pre amps. I know this is complicated with PC boards but I can do it or simply replace the PCB with another circuit.

However, there is no sense in doing this if no one is interested in assisting me. I can't make this work myself. I've posted two questions on this amp and gotten no answers to those questions.  Don't know why. If you know why- I'm boring or a jerk or maybe the questions I ask are uninteresting, please say so. I can live with the truth. I come to this Forum as an inexperienced student to learn, and have gained an immense knowledge of tube amp circuitry. Still a student though and that will make finishing this project myself much harder.

So, continue as a new Topic- "Dual channel amp with ab763 Output"? Speak your mind- Rant, Rage, Advise, Suggest. "Silence is Golden" only when your adversary is headed for destruction.

Best regards and thanks for all the previous assistance,

Silverfox.