PAS 3 as guitar preamp

I've been converting a Baldwin Model 45 Organ amp into a guitar amp. I re-did the 4 section electrolytic can in the corner with separate electrolytics and the 0.1 uf coupling caps. Those two measure made it sound 100% better.  All the resistors checked out good so none needed to be replaced. An electric guitar doesn't have much output, maybe about 150mV, so I grabbed a restored PAS3 to use as a preamp. It actually works really well, luckily it still has the tone controls and loudness, the phono input has plenty of gain, it can feedback if turned up a bit, but also sounds really good clean. Eventually I want to add a couple of gain stages maybe a couple 6SN7, and a spring reverb and a vibrado.  Maybe even this circuit. : http://www.rfcafe.com/references/popular-electronics/build-your-own-vibrato-dec-1957-popular-electronics.htm

Today I brought it up on the variac for the first time and monitored the voltages. At 117VAC line voltage the filaments were 6.28V and the B+ was 392 VDC, right on the money per the schematic. It's been running about 4 or 5 hours and the giant power supply transformer is just slightly warm. Seems like this thing is over built at about 40 Watts.  I decided to not replace the two tall 4 uF 600V e-caps, I think they are the sealed liquid type and seen fine. Why the heck does it have those 600V caps when the B+ is fairly low at 400V?

Any one have any information on the PS transformer, it's absolutely huge, part no, C512-14542-F. I need to know if it can power a second chassis. Here are some pictures. Also, check out how big that choke is, it's bigger than the output transformer.





The two speakers are 12 inchers. anyone have any idea what their impedance might be? It's gonna sound even better when I build a speaker cabinet. I'll use wood salvaged from the original organ body. The amp and premap/reverb chassis will be in a separate cab.

Are those Hallicrafters  in the background?
They sure bring back memories....Nice variac too...

Bally wrote:Are those Hallicrafters  in the background?
They sure bring back memories....Nice variac too...

I see a Hammarlund receiver and a Hammarlund speaker on the bottom underneath and a Yaesu FT-101 with a Hallicrafters speaker on it. Tube ham gear, tube audio gear and guitars. I like this guy already.

The 600v cap rating when the HV is only 400v is no problem. Back in the day overrated components was very common. "Built like a tank". These things were not meant to be "consumed" for a year or two and thrown away, they were meant to last a lifetime!! In electronics school we learned that if a component was expected to see a maximum value of 1 amp, use a component that can handle 2 amps! Overrating by 50 to 100% was expected in a solid design!

Sorry I can't help you with the transformer question but it does look fairly hefty!!

Rock on!

Why not try a guitar preamp pedal?

Yaesu!



The larger caps are a good thing ... be interesting to see what sort of surge you get when powering up the amp that they felt the extra capacity was warranted.

Same thing with the large transformers and chokes ... can never have enough of a good thing.

PS ... the extra large choke and coil might be because of the speaker type. Two of the old school speakers in mono would need em ...

Nice compact Shack you have there sKiZo. Do those Christmas light throw off any RFI on your receivers?  Good point on the surge voltage, I'll measure that next.  Come to think, of it I need to do a thorough voltage check on all the tube pins for my restoration log book.

The B+ takes two paths out of the PS transformer, one path to one of the 4 uF 600V oil filled caps, then to the large choke which is grounded, unlike our Dynaco amps. The other path is to the second 4 uF oil filled cap and then through a few large voltage dropping power resistors before leading  into then the 450 V electrolytic can that I replaced.  

Another interesting feature of this amp is the use of R.F. chokes off the plates of the power tubes, apparently to eliminate parasitic oscillations. Over all the chassis is very simple and sparse underneath. I'll post a picture of the underside when I measure the surge voltage.

sKiZo wrote:Yaesu!



The larger caps are a good thing ... be interesting to see what sort of surge you get when powering up the amp that they felt the extra capacity was warranted.

Same thing with the large transformers and chokes ... can never have enough of a good thing.

PS ... the extra large choke and coil might be because of the speaker type. Two of the old school speakers in mono would need em ...

aaaahhh...10-4 good buddy....whats ya 20??

I've done a bunch of work on the cabinet and have it pretty much figured out. Just a few details to work out. I thought I would comment on the 600 VDC ratings of the first oil filed caps. Turns out they see 540VDC on initial power up. I guess that's to be expected with directly heated cathodes on the dual 5U4G rectifiers.  Here's a scan  of my tube voltage readings at 115V line voltage.

Check out the RF choke coils off the power tubes. Never seen those before.:



Rear view picture of cab. Everything is glued and screwed.  The cabinet alone is very heavy, good thing it has the original grab handles. I'll report back later on the weight of the amp  and the cab. but sure to be gross.  


This is a front view of the guitar amp. The top and back are being constructed. So far,all the wood and fasteners have been salvaged from the organ. Very good quality wood parts I must say.



That's the update for now.