Grommes Precision Electronics 15W PA conversion

[bmccowan]

When Amber B was working on the Lafayette PA conversion, I mentioned this project I was just starting, and the topics of 6SJ7s, 6SL7s and other octal preamp tubes were discussed. I got back to this project and finished it last night. It started as a 15W PA with mic and phono inputs sharing a single pentode preamp. 6SJ7 > 6SC7 > 2x6V6 - 5Y3. The chassis had lots of room, so I punched for another octal socket. The wax caps had to go and most of the resistors had drifted way off, so I stripped most of the circuit, cleaned up the sockets and added a ground buss. I like the control panel so wanted to stay with the 2 vols & single tone pot. I added a 6SL7 and wired it parallel, using mixing resistors off the two volume controls. I set up the 6SJ7 with Gibson GA-30 values, and used a Valco style PI with the 6SC7. I tried both a Gibson and a Valco large beaver style tone control, before settling on a Framus style mid control. The output transformer was NFG so I replaced it with one from an AO-43 Hammond organ amp. It was a fun project and sounds great - lots of tone variations mixing the two channels and adjusting the Framus mid-control.
A couple of questions:
1. I often read complaints about microphonic octal tubes. Including in Amber's thread. I just do not get it. I have built many all octal amps and repaired/restored many old Gibsons, Valcos, and converted a bunch of octal PA amps. I run into microphonic issues occasionally, but do also with those wimpy little 9 pin jobs. I think octals get a bad rap. And I think the reasons the industry moved to minis were economic, not performance. Less glass, less metal, more tubes in a shipping case, smaller chassis requirements etc. Same kinds of decisions all industries make to cut costs and improve margin. I'm wondering what others think?
2. Should the mixing resistors always be the same value. And is the value a critical factor. I used 100K, but the Plexi 6v6 uses much higher values (470K) if I remember right. And I'm surprised that having a pentode and triode channel isn't done more often - seems like an easy route to tonal variety? Sluckey's Dual Lite with an EF86 and a 12a_7 channel is a good example, that gave me  confidence that this would work out ok.
3. I had to replace the OT anyway, but it was pretty small and had many output taps, in addition to the usual 4,8,16 it had 250 and 500 ohm taps. Are those for a distributed speaker system for classrooms and the like? I'm curious if having the extra taps affects OT performance or if its just tapping different winding points and doesn't affect the sonics for the 4 and 8 ohm taps? I did google up some OTs with those secondaries, but remain undereducated as to their use. Any knowledge you can pass along is appreciated.

[PRR]

> the reasons the industry moved to minis were economic, not performance.

Original idea was better performance.... above 30MC (MHz). Less parasitic L and C.

And yeah, it cost less.

Hi-Z speaker windings are not necessarily for "distributed" but for loooong lines. When you need a long wire, cost favors thin wire. Past 100 feet you tend to have as many Ohms in the wire as in a 4/8/16r speaker, and lose half your costly amp-power in wire. Lever-up (and back down) to hundreds of ohms, you can use real thin stuff over very long distances.

[bmccowan]

Thanks PRR,
The long wire lines had not occurred to me, even though I'm very familiar with voltage drop over long lines. I think back on schools (I started school in 1958) and how every school system (although I actually started in a one-room schoolhouse) had one of these tube PA systems, not to mention several Filmosound projectors with now sought after tube amps. Imagine what is buried in landfills!

[trobbins]

Was there a reason you replaced the OT?  I'd suggest for guitar amp use, the OT configuration would be just fine - just stow the 250 and 500 ohm taps so they can't couple signal to anything nearby.  For guitar frequencies, the OT is likely to support an uprated output power as well.  The top cover of the amp looks spiffy, although it could get hotter underneath if you uprate the output stage valves (PA amps typically operated with low 'duty cycle', and lowish idle power, so temperature rise may not originally have been too concerning).

[bmccowan]

trobbins, thanks for the input. I should not have assumed that everyone would recognize "NFG" which is a term describing failed parts. I picked it up decades ago from plumbers and electricians when I had a solar contracting business.

The output transformer was NFG

(no freakin good.) It had a short. Of course I did the same with another term, "large beaver" which is the name a pedal clone company gives to the "Big Muff" circuit that is used in the Supro Thunderbolt.

The cover has many more vents in the top and side and is mostly open in the back. But its a good point and I'll watch for high temp as I play it more.

[trobbins]

Getting any awareness of a failure can be helpful - sometimes other collateral damage along with the type of OT failure can be insightful.

I seem to find that every new PA amp conversion raises interesting technical issues and design decisions.

[bmccowan]

It was returning no HT to one of the power tubes. winding shorted to the core I presume. One of the power tubes was likely original the other clearly was not - likely related. First dropping resister was roasted, but it was a 2W resistor, so not too surprising.
I find the old PAs to be a fun way to experiment with different circuits. When I repair guitar amps I pretty much stick with original circuits, making the amps safe with good sound. And if building from scratch, I tend to plan carefully rather than experiment with the completed circuit.
I have two Bogens that came from a square dance caller that are awaiting conversion, but I picked up one of the early all octal GA-40 Les Paul amps that needs a lot of work, and that should come first. 

[AmberB]

I do like the old Bogan PA amp chassis!  :-)
I've converted a couple of the CHB35As and one CHB33 to guitar amp use.  That Lafayette PA amp I just converted was a bit of a departure for me with it's octal tubes.  It's been an interesting project...

[bmccowan]

Amber, did you get that Lafayette working well with your 6SL7s? One of my favorite tubes!
I also have a nice Don McGohan PA waiting for a conversion. Its a MG-16. 6Au6, 12Ax7, 2x6L6, 5u4GB. Got it with the nicest coke bottle RCA 6L6s!! Hardly looked at it, but I think it is either parallel SE, or self split. Has great bones.

[AmberB]

Yes, the Lafayette amp worked the way I expected it to once I put 6SL7s in it.  I haven't finished it yet because I haven't decided if I want it to have 2 inputs with volume controls for each input, or if I want a post tone circuit volume as well as the input volume.  The thing has 4 controls, 2 for the tone controls and 2 volume controls available.

[DummyLoad]

bmccowan, I have a McGohan PA with a similar tube lineup. 6J7, 6SC7, 6L6G & 5U4G. I like the plan you presented so I took the liberty to tidy up the schematics and fix what I believe is an error - R5, a 2.2M g2 resistor, you show is connected to the plate - It should be connected to B+. I hope you don't mind me taking the liberty to do so, and If you do then please let me know and I'll remove the attachments.

How much of what you did is original to the Grommes and what did you change? Obviously the Framus mid ckts aren't. 


Lastly, the values of the pots are not indicated in your schema, so I penciled in 1MA, but based on similar amps of that genre, they are likely to be 500KA? If you know the value, please share.

--Pete