Peavey Roadmaster 160 Watt Tube Amp

[CJ]

a cargo hold on a 737 popped open and this thing fell through the roof and landed on the bench,  built tough but needs some work,

 

[CJ]

orig Peavey tubes except one, has a good rep online, you know, "wish i hadn't sold my Roadmaster" comments,

 

[CJ]

pwr transformer was possibly tweaked for hum reduction,

Peavey always had he big iron on it's hip, check the OPT, hernia box, 

four 12AX7a and two 12AT7 under the screen cage thingy, foam on the top sits on the tubes, same deal as the Mesa .22, hmm,  wonder if Randall ripped it,

 

[CJ]

what's under the hood?  glad you asked,

looks like a good build, i wonder if JR was in on this, a cut above most Peavey stuff you see,

that blank circuit board on top of the preamp board is a copper shield,

 

[CJ]

lots of fuses inside,

check these weirdo ones>

 

[CJ]

never seen this connector used in a  git amp,

looks like a telephone switchboard connector, great idea, easy to service cables,

 

[CJ]

parent dir

http://schems.com/manu/peavey/peavey.htm

schemo

http://schems.com/manu/peavey/peavey_roadmaster.pdf

uses 5 opto's for channel/reverb switching,  VTL5C3

data sheet here>

http://www.alliedelec.com/images/products/datasheets/bm/PERKINELMER/70219697.pdf

opto circuit> clever, CR3 and CR4 add enuff voltage drop to persuade current to go thru the other string when the pull/select sw is engaged,

you can check opto on resistance with an ohm meter across the device, make sure you don't get on the voltage side with the ohm meter,

 

[CJ]

found an open 220K plate resistor so the amp is working but does not sound that great, so we are replacing all the coupling caps and upgrading all the plates resistors in the preamp, sounds better, but some harshness, probably from the really worked output tubes, but there are 6 of them. mucho $$$ to replace all,  so we are rebuilding the phase inverter to see if that helps, output tubes sound alright when biased hot, but then the tube sockets start melting,

do we really need the cathode follower 2nd inverter or can we drive the 6L6 grids with just one 12AX7a ?

here is the inverter circuit, changed it into standard long tail with 82K/100K plate resistors and 10K in the cathode instead of 22K,

will  bypass the 2nd inverter  to see if we get better sound,

Marshall gets away with driving 4 grids with the single inverter so i bet we can drive 6,

 

[PRR]

Book value of grid resistor for 6L6 in fix-bias is 100K *MAX*.

So three per side, the spec limit is 33.33K.

As we know, geetar amp makers often pushed the specs. This isn't battle-ground communications, nor Aunt Matilda's kitchen radio. If the extreme spec tube melts, the player will replace it.

So you should have R73 R77 at 33K, you have 47K, leave it be.

You want the tube plate resistance to be much (2X-5X) lower than the load it drives. So you want the driver Rp to be 10K or 25K.

12AX7 is 60K.

12AT7 at higher current is 25K. You find this in some of the big Fenders.

12AU7 can be below 10K, but the gain is not there. You must go to a more Williamson-like design with another gain stage before the hard-worked 12AU7. This has never been popular in guitar amps.

So 12AT7.

You want the plate resistor to be a split-difference between the 47K load and the 25K plate resistance. Pencil 33K. However Fender used 47K here. I'd go with Fender's value.

There will be over 200V drop in the 47K plate resistor so 2W minimum. Two 100K 2W parallel would not be over-kill.

12AT7 has mu of 60, Rp like 25K, and is driving (47K||47K)= 25K load. Per-side gain is 30. The long-tail splits this to each side so the gain from the previous stage to the power tube grids is 15.

Bias is 58V and peak swing is probably about the same. 58/15= 4 Volts peak into the driver stage. I won't review an entire Peavey plan to see if it can do that. With 475V supply and over 400V to smaller tubes, it probably will.

 

[PRR]

BTW.... repeating the calcs with 12AX7, the higher mu of 100 is defeated by the higher Rp against this very low load impedance. Where 12AT7 gave gain of 30, 12AX7 gives gain of 29.4. So not a huge difference. However the beefier 12AT7 will take the high heat and will pull-down better than the over-leveraged 12AX7. Since the values and pinouts are the same either way, you could just build it and tube-roll for a preference, and to confirm that in a panic the other type will get you through the night.

 

[CJ]

thanks PRR! saves me a month of rehashing RDH4, 

yeah Fender is pushing it with those 220K grid resistors in the twin Reverb, (quad 6L6 amp)

this Peavey inverter circuit does have a good Presence circuit, that cap in series with the pot gets rid of the pot scratch and reduces noise quite a bit also,

changed it to a 1.5 uf non polarized and it sounds great,

also changed 100K feedback R to 47K like Marshall uses,

this amp borrows the Ampeg SVT grid stoppers, using 47K instead of typical 1.5K,
helps blocking distortion and gets rid of a little harshness as high end roll off/Miller C comes down a bit,

how much gain will I lose if i bypass that second 12AT7 circuit?

47K plate resistors are  used in Fender Pro Reverb inverter, which is a good amp, so we will give that a try,

Macintosh also used the two tube inverter circuit, wonder if Peavey borrowed this concept from them,
looks like Fender also has the cathodes in the inverter sitting near the spec limit,

page 138 has some stuff on inverters, Crowehurst  Hi Fi Circuits>

(use fullview to read)

https://www.scribd.com/doc/59036351/Hi-Fi-Circuits-Crowhurst

 

[CJ]

ok we got this thing sounding alright, left the 2nd stage inverter in there along with the first stage where we changed the 220K plate resistors to 82K/100K Marshall/Fender style and changed the 480K/100K grid resistors to 1 meg.

did a full on max volume test, man this thing is loud! but saw some smoke, it was the dropping resistor in the pwr supply that feeds the screens, so we changed screen resistors on the 6L6 tubes to 390, ripping out the 100 ohm stock resistors, still a little too much heat on the dropping resistor, old tubes must be drawing excess screen current, so we added a Fender Dual Showman  choke in place of the 400 ohm 10 watt Corning pwr resistor, no more smoke,

no room on top of chassis, so we shoe horned it inside>

 

[CJ]

another mod we did was change the 10K cathode resistor on the second stage 12AX7a overdrive preamp, put a 2.7K in there, 10K seemed weird, some type of neg feedback scheme?  maybe a cutoff distortion tweak?  not a great sound, that cutoff distortion,
2.7 K cathode resistor got rid of a little squealing when all 3 gain controls in the overdrive channel were maxed out,

good thing this thing has a fan, makes a good space heater, 
bias set at -58, tube sockets start to gas at -50,

bolted it up so we can work on something else. like an old Acoustic,

 

[PRR]

> 10K seemed weird

This is taken from Marshall. Marshall stole the 5F6a plan, then wanted a little more gain and some more-obvious clipping. The 10K starves the 12AX7, also huge local NFB, so low gain to a point and then it clips like a steel hammer.

It's a sound, but it may be a bit much at 160 Watts.

Like the Ampeg V-series amps, tastes have changed since those days.