Rauland-Borg 1916 PA to guitar amp

[punkykatt]

Hey Guys, any of you converted one of these amps to guitar?  Looks like a good candidate for a 5E3 but has these octal pentodes before the dual triodes. Any ideas, suggestions, schematics , layouts are  much appreciated.  Thanks in advance  Punky

[kagliostro]

May be a Gibson BR-6F or Kalamazoo KEH ?





Adding a rotary switch with cap you can have a Tone Control that is missing

give a look to old Gibson schematics, they often use 6SJ7 tubes

here a large collection of shemes

http://el34world.com/charts/Schematics/files/gibson/Gibson%20Amp%20Master%20Service%20Book.zip

you can find other Gibson schematics on Doug's database (where is also the previous .pdf)

http://el34world.com/charts/Schematics/files/gibson/Gibson_Schematics.htm

Ciao

Franco

[PRR]

1916? Isn't that a little old for you?

Does it work? Are the tubes good?

You can play, stock, low-gain at Phono or hi-gain at Mic. Just adapt jacks.

I don't think the stock circuit has any magic mojo. The plate-mixer at V3 V4 looks dubious; and pointless in a one-guy application.

6SC7 is a low-spec 12AX7 with both cathodes tied (which rules out several popular stage plans). 6SQ7 is a half-12AX7 plus ignorable diode. 6SJ7 is a sweet audio pentode, or fabulous audio triode, maybe too low-gain and too-clean for guitar flavor.

TheTubeStore has 6SC7 at $20-$50--- more than 12AX7 but maybe less than 12AX7 plus socket plus filler-bracket, so not a wrong choice.

[punkykatt]

Thanks for the replies guys.  1916,   my father was born that year, he would have been 100 last April.  .   The amp plays but no magic mojo as PRR stated. Its going to be a while before I get to dig into this project.  Thanks again for the info.  Punky

[PRR]

> my father was born that year

Sorry to stir the memory. My father is getting along and I know I will miss him.

It does sound like A Sign. A tribute amp for your father.

As a rather plain amp with minimal re-working: disconnect all but heater on 6SQ7. Disconnect the dubious high-cut "Tone" control. Connect both grids of (my) V3 together to a volume control (the Mic 1 pot is fine). Wire 6SQ7 as a conventional Fender input stage. Use a standard Fender B(M)T tonestack to connect 6SQ7 to the volume control to V3. Fix the leaky caps and drifty resistors. Input and output jacks to taste. 4/8/16 outputs a real bonus. Power out about 15 Watts.

A balls-out heavy rebuild: SS rectifier, 500V main caps. 6L6 outputs. Fix-bias, trim for around 40mA each at idle (to stay near the original current). Rebuild V5 as a Fender long-tail, NFB per Fender practice (you may have to swap plate leads?). Do the rest of the amp in Fender-Marshall style, using 6SC7 both-sides-parallel or 6SQ7 as "half 12AX7". Re-wire former 6SJ7 sockets as needed. Closer to 25W output.

[trobbins]

If you do keep working on the amp, and enjoy tweaking the performance, then it could be worth looking at how the output transformer speaker tappings are made.  On some transformers it is practical to disconnect the winding wires coming in to and out of a tap.  On your amp the tap of interest is the 16 ohm tap - if you test the output voltage on the two windings (0-16 tap, and 16-250 tap) and they are the same then they have the same number of turns.  Those two windings can then be connected in parallel for a bit more technical aesthetics.


A working amp also needs a little self-protection, such as a PT CT fuse, some speaker open-circuit avoidance measures, and add some grid and screen stoppers as per most amps now.

[punkykatt]

Thanks for this great info guys.  PRR, spend as much time with your father as possible.  Forever is along time.

[punkykatt]

Hey Guys.  I did  a turns ratio test on the output transformer and was surprised with the results.  I wish they had marked the  transformer wire colors on the schematic.  This is what I did.  Pulled the power tubes, disconnected the CT and 5.6k NFB resistor,  applied 10vac to  the common and each of the other secondaries  and took AC voltage readings on the primaries across pin 3 of the power tube sockets.   Using the ST-700M socket numbers listed on the schematic  here are  the voltage readings:  1= common, 2= 48vac,  3= 267vac,  4= 60vac.  5= 374vac,  6= 529vac,  7=68.5vac.   The formula used to get the primary and secondary Z is: ex.   take primary volts from #5   374 divide by 10= 37.4 squared = 1398.76 x speaker ohms to be used 8 = 11,190 ohms primary with a 8 ohm speaker or 5,595 ohms with a 4 ohm speaker.  This test results are way off from what is shown on the schematic? Perhaps schematic error or assembler error?  Any how,  which primary Z would be best to run 6V6`s @ 365 plate voltage  the 11k or the 5.5k?  Thanks in advance for any input.  Punky

[kagliostro]

I think 5.5k is better if you have 365v B+


Franco

[PRR]

I can't make sense of that. The pin numbers are meaningless, list by nominal impedance rising.

10vac to  the common and each of the other secondaries  and took AC voltage readings on the primaries across pin 3 of the power tube sockets.
1= common,
3 (4r)= 267vac, 
4 (8r)= 60vac. 
6 (16r)= 529vac, 
2 (250r)= 48vac, 
5 (355r)= 374vac, 
7 (500r)= 68.5vac.

Maybe I have mis-transcribed, but something is wacky. 10V at secondary in order of rising impedance should show declining voltage. The 4r and 16r do look right, but then "8r" can't be right, and the others look scrambled.

Am I misreading the impedances, the pins, or your notes?

BTW, the right answer is 8.6KCT. This is from 344V plate-cathode and 80mA current, doubled. 8K would be very fine also. 5K is some low, 11K is rather high.

[punkykatt]

PRR , Pin 3  and pin 1 of the ST-700M connector socket, 10vac applied  gave a reading of 267vac on primaries. If I used  the 4 ohm load shown on the schematic in the calculation would give a 2.85k primary Z.  Too low for a 6V6GT I would think.  A 8 ohm load would turn out 5.7k primary and a 16 ohm load would turn out 11.4k primary.   I think the person who wired up the secondaries to the ST-700M socket did not follow the schematic?  My question is what primary Z will the 6v6s be the happiest at 365 plate voltage?

[PRR]

> My question is what primary Z will the 6v6s be the happiest at 365 plate voltage?

I thot I answered that? "the right answer is 8.6KCT"

[PRR]

----------------------V ratio---Z ratio---@8600--@6600---@11,5K--Prob Ohms
1= common,
6 (16r)= 529vac,----53:1---- 2800:1------ 3------2-------4-------- "4"
5 (355r)= 374vac,-- 37:1---- 1400:1------ 6------5-------8-------- "8"
3 (4r)= 267vac,---- 27:1------712:1------12------9------16------- "16"
7 (500r)= 68.5vac. 6.8:1------ 47:1---- 187----143-----240------ "250"
4 (8r)= 60vac.------- 6:1------ 36:1---- 240----184-----332------ "355(70V)"
2 (250r)= 48vac,---- 5:1------ 23:1---- 344----263-----500------ "500"

OK, they aimed at nominal 11.5K-11.7K to leave room for sloppy installers who might hang loads of 3/4 of design impedance. (Not accounting for what seems to be a major factory mix-up, so that NONE of the taps gives rated power, and only the 500/250 tap is close enough to be overlooked.)

You could hang 6ohms on the (corrected) 8ohm tap, etc. I doubt there would be much difference, even if you could source *identical* speakers in 6 and 8 ohms.

Hanging 4 on the 8 tap (6KCT) is not absurd for a guitar amp. Will it be louder? Sweeter? I don't know. Try and see.

[punkykatt]

Sorry PRR, sometimes I look but don`t see.  Thank you and Franco for the input.  Much appreciated.

[punkykatt]

I started checking out some of the Gibson 6SJ7 preamps. Right now I have the amp wired up using only the top circuit (one channel)of  the Gibson GA 30. I have the tone control wired just like the schematic is drawn.  The tone control does nothing.  Are any of you guys familiar this tone control setup?  Thanks in advance .  Punky http://el34world.com/charts/Schematics/files/gibson/Gibson_GA-30_65J7_PREAMP.pdf

[PRR]

> wired just like the schematic is drawn.

That schematic can not be right.

Grid has no DC leak path. Caps/pot have nothing to work against.

I'll speculate it needs a ~~200k-1Meg grid to ground.

[punkykatt]

Thanks PRR, tomorrow I will clip in my resistor sub box and see what values works best.

[punkykatt]

Well I did not like  the Gibson circuits i tried with the 6SJ7`s.. No balls  No butter.   I set the amp up like the Tweed deluxe  5E3 using 6SL7`s for preamp and PI.  For the bright  channel I paralleled one of  the 6SL7`s.  Holy  moley what a difference.  I brought it to the owner at his studio.  He plugged it into his cabs.  Dang,   from clean to scream to Marshall stack sound with just turning amp volumes/tone and guitar volume/tone.  He is in tone heaven.  He says he is going to take it on tour this October.    Just thought I would share  my experience.  Thanks to all on this forum.

[kagliostro]

EH, NO ! 


Adesso vogliamo uno schema delle modifiche !!

Franco


ps: 


and BTW I didn't see any 6SL7 on the original schematic (??)

[Oscillating]

I got one of these. The easiest way to enjoy this is to simplify to a single 6sj7 into the phase inverter and rock out. Plenty of gain for crunchy rock. The 6sc7 into the PI sounds real old school, but not as good. 4,8,16 ohm switch out, and you could switch the NFB.

[Lectroid]

I'm a little late here, but...

I modded a Bell & Howell 179 6V6 projector amp into a decent 5E3 clone with nice smooth breakup.  Neil Young would approve.     I liked the 6SJ7 for a first stage, but I do like the 6SL7 too. 

Here's my schematic.

[scstill]

Here is what I did; seems you have enough for this type of conversion
Channel one 6sc7 (like deluxe 26/5c3)
Channel two 6sj7 (like gibby br6f)
6sc7 as PI into 6v6 in PP
Housed in a 52 Packard car radio cabinet
Schematics and pics here
https://stillampd.com/black-and-blues-05

you could go one up with that 6sq7 in a third channel

[PRR]

> 52 Packard car radio

No CD tuner marks?

A Plymouth in a nearby thread had those. Not having them seems somehow odd. (Yet another thing the youth of today will never know... like how to replace a typewriter ribbon, this week's project here.)

https://www.rfcafe.com/references/radio-news/conelrad-radio-television-news-april-1955.htm

[Lectroid]

CONELRAD!  Oh, yeah.... That brings me right back to those halcyon days of 'duck and cover' and lining up for polio shots and I Like Ike.  Around 2002 I took an upper-division college class on The History of the Fifties.   As a cultural period, it was even stranger than I'd remembered.  Feminism, conformity, and racism, all bubbling right below the surface, just waiting for the Sixties to ignite and set off the counterculture and ... ahem ... rock 'n' roll.

[shooter]

....and now the 2nd verse, same as the 1st, a little louder and a little bit worse 

[DummyLoad]

Here's my take on it - What I'd start with anyway. Did away with the 10R/5.6K NFB loop - you can leave it if so desired.

Brite channel is higher gain. Blend the two. Mixer forward is not changed much, 6SQ7 not used.

That thing most likely needs a cap job - C1-C3 is a CE Manufacturing 1-3/8" can 40/20/20 @ 500V  p/n C-EC40-20-20

Attached the .SCH draft file for you to edit if you wish.

--Pete

[Oscillating]

This is my first project. Trying one tube as preamp at a time. About to set up a channel as cathode biased 6sj7. Then I got a 6sl7 to try out. Might end up with 4 channels of pre flavors. But, I haven't started running them in series yet, and that may end up going a whole other direction.
I recapped it first. I got rid of the can and built a filter cap section where the mic transformers were. My approach was to go bare bones, so I scrapped the 6sc7 mixer tube, the tone circuit, and NFB. I was expecting a very mellow amp. This makes very crunchy rock'n roll tones when cranked. And the amp has an "old" sound character to it.
Im looking forward to staying alive, keeping all of my fingers, and learning about setting up tube circuits.

[Lectroid]

....and now the 2nd verse, same as the 1st, a little louder and a little bit worse 

no, we didn't start the fire