What power amp is this??

[Toxophilite]

Anybody know what guitar amp this power amp came out of??
Still have to test the rusty transformers. It was thrown in with a lot of stuff and clearly has been monkeyed with
I figure I'll probably at least get a choke out of it
But I was curious if anybody had an idea. I was told it used an unusual and expensive power tube that started with a 7 or 73. The board looks 70s to me
Update: the OT has 2 secondary taps (4 and 8 ohm)with 33-1 and 23-1 turns ratios
The Pt measures 250 VAC across the red wires (not hooked to anything I don't trust the filter caps  . for secondaries there are 2 red and  2 yellow wires as is standard, 2 blks on the primaries  and 2 giant solid copper wires as heater wires (clear plastic insulation)

[eleventeen]

Hmm. Maybe the amp used 7355's...Somewhat similar to 6L6 EXCEPT the basing diagram is different...enough different so that something would almost certainly blow up if you stuck 6L6s in an amp wired for 7355. Offhand, though, similar ratings to 6L6 power and fil current wise.  Plates are pin 3 for both tubes, but 7355 uses one or more of the "blank" pins for 6L6 and it looks like 5 is the cathode and 6 is the control grid. Very different.


http://frank.pocnet.net/sheets/135/7/7355.pdf


I *do* see blue-brown wires to pin 3 of each output tube. Going nowhere at present. Both tube types use pin 3 as plate.


The idea that a pair of the black caps are opposite facing hints at a series connection for those caps; eg, 500+ volt PS probably with a voltage doubler rectifier.


What's the voltage rating on the yellow cap? If it's 600 volts, doubler-type supply.

It almost looks Gibsonish but that 's a vague guess.


Some beefy iron there, that's for sure.


Agreed, I'm not sure I would trust the thing to plug it in. (without lamp limiter, face shield, remote AC switch so's I could stand in the next room behind the blast shield. 

[Toxophilite]

Cool I looked up 7355s (I was looking at my tube tester tube chart for power tubes that started with 73)
I should've checked their pinout
The filter caps  are:
Big Sprague Atom - 100uf  350vdc
then
50uf 350vdc
100 uf 150 vdc
50 uf 350 vdc


There's been some hacking and moving things around near the PT. I would be much more comfortable seeing the original schematic and getting it back to normal if I was to hook it all up. I think the transformers are good

[sluckey]

That looks very home made to me. No telling what it was meant for. The RCA phono jack input does not make me think of guitar. I'm thinking hi-fi power amp.

[Toxophilite]

Musician heal thyself!


I recontacted the guy I bought the amp off of and now he knows what the amp is, I guess he checked his records(he used to have a music store) he claims it was a Randall 800 series amplifier that used 8417 output tubes which I've read can be substitued with 6550 if you adjust bias.
Which would account for the big iron (Marty Robbins!)..hmm I wonder if I want a giant Amplifier...oh I already have one!!!


It looked a little home made to me too as it has a rating and model number plaque on the side which is completely blank!

[Toxophilite]

Looking up Randall amps this cracks me up!


http://www.randallamplifiers.com/downloads/SATAN-MANUAL_120314.pdf

[eleventeen]

Even without your "Randall" hint....I started to think it *could* be or have been the power section of an upstairs/downstairs guitar amp. Indeed, I am trying to think of any kind of power amp...organ; theater; jukebox that DID NOT have at bare minimum a user adjustable volume control. Not many. Maybe some Heathkits/EICOs. Dyna Mk III of course.

[DummyLoad]

randall rrm 1-100

http://music-electronics-forum.com/t33043/


looks like it's been hacked: V doubler PS and the yellow cap is a hack. there were 2 caps on the PCB and a bleed R.


the output transformer looks like it's one out of a norlin/gibson SG-100

--pete

[Toxophilite]

Bingo!


That looks like it, slightly different OT but everything else identical, down to the numbers on the transformers which start with PT on the ..PT and OT on the OT
Sadly it looks like that guy can't find a schematic
But it's good to know what it is!

[DummyLoad]

Bingo!


That looks like it, slightly different OT but everything else identical,

looks like the OT was salvaged from a gibson/norlin SG-100


--pete

[Toxophilite]

I'm not sure about that
Maybe they had the same dimensions as far as mounting holes as there is only one set and the chassis is bright shiny metal where is was mounted(it's currently detached) It also has the same sort of designation numbers as the PT
Thanks for tracking it down though. The seller claims to have a schematic as well as the original tubes which he just dug and will sell for quite a good deal..I don't know if I need a 100 watt amp. might make a good small bass rig.

[DummyLoad]

>> I'm not sure about that

i am - i have one. SG100 uses 8417 - same era as well.


your amp has been heavily hacked. PS looks similar to the SG100 as well...so does the driver.


--pete

[DummyLoad]

i salvaged the PT & OT out of a SG100. the OT is bad - has some shorted 1/2 primary winding where previous owner put a 6L6GC in and pulled too much current. it starting red-plating with 8417 connected to that 1/2 primary after that as well. we found another SG100 in rough shape and salvaged that chassis. we took a couple of knobs and pots of this one.

pics below, compare measurements to be sure. just for curiosity's sake. :-) 

--pete

PT dims.:

[DummyLoad]

here's the OT pics.

OT dims.:

[Toxophilite]

The reason I wasn't sure was in that link you sent me of the randall amp, it had the same sort of part numbers on the transformers.


POwer transformer - PT1523 (I think )
Output transformer  - OT542?


anyway dimensions in inches and they do seem bang on (I don't have a micrometer...I'm envious!)


PT 3 3/4  by 3 1/8


or 3.75  by 3.125


OT 3 3/8 by 2 13/16


or 3.375 by 2.8125


Perhaps the two companies got transformers from the same source?

[Toxophilite]

Sadly for the original seller..he just emailed and said he dropped one of the 8417s...ooops!
I guess sad for me too as he was going to sell them for $50 for the pair but this is a little more amp than I can use and I've read with rebiasing you can sub in 6550s or Kt88s maybe?
I might pass this on to a friend to build into a practice bass amp...maybe I'll steal the choke first!! haha

[DummyLoad]

Sadly for the original seller..he just emailed and said he dropped one of the 8417s...ooops!
I guess sad for me too as he was going to sell them for $50 for the pair but this is a little more amp than I can use and I've read with rebiasing you can sub in 6550s or Kt88s maybe?
I might pass this on to a friend to build into a practice bass amp...maybe I'll steal the choke first!! haha

if they are indeed similar, then the bias winding is probably 25VAC: measure yours? i think it's the yellow leads. the red leads should be 220VAC for the B+, the green (white?) are filament, and the black primary.


scrap the PCB and build an eyelet or turret board.


i don't see OT connections on either 8417 sockets, nor do i see OT CT connected to B+ - are they hanging outboard in the first post top pic? 


--pete

[Toxophilite]

The Ot is disconnected, you can just see the stubs of the brown and blue wires on pins 3 for both sockets
I had the OT sitting on top of the chassis for the picture.I think there was plans to resue things that got shelved 1/2 way through. The OT seems to test okay
I get 250 between the red wires and around 53 between the yellows
Are your filament wires great thick solid copper wire? ( like house wiring) cause these are, right out of the PT insulated with a clear plastic

[sluckey]

Those numbers for voltages seem right , are your filament wires great thick solid copper wire( like house wiring) cause these are, right out of the PT insulated with a clear plastic

That was common practice a long time ago. Many manufacturers would just bring the ends of the actual filament winding outside the shell rather than solder a stranded 'pigtail' onto the solid winding wire inside the shell. I have a couple PTs like that.

[eleventeen]

Kinda thought it looked Gibsonish....


Anyway, if you compare 6L6 and 7355 basing diagrams, you'll see that installing a 6L6 in a socket wired for a 7355 (and no doubt vice versa) will create huge issues. That's probably what smoked the OT. It won't just "not work", it's almost as if this is a deliberate mfr trap for those who would do so. If 8417s are the proper output tubes, then those have the same basing diagram as a 6L6.


The chassis look usable for something or other. Both trannies are standup so you don't have giant chassis cutouts. 8417's use 1.6 heater amps!

[DummyLoad]

Kinda thought it looked Gibsonish....

Anyway, if you compare 6L6 and 7355 basing diagrams, you'll see that installing a 6L6 in a socket wired for a 7355 (and no doubt vice versa) will create huge issues. That's probably what smoked the OT. It won't just "not work", it's almost as if this is a deliberate mfr trap for those who would do so. If 8417s are the proper output tubes, then those have the same basing diagram as a 6L6.

The chassis look usable for something or other. Both trannies are standup so you don't have giant chassis cutouts. 8417's use 1.6 heater amps!

you missed the part of the thread discussion where it was surmised that this positively is a 8417 based power amp. 6L6 pattern can also be determined by looking closely second pic of the opening post.


--pete

[DummyLoad]

The Ot is disconnected, you can just see the stubs of the brown and blue wires on pins 3 for both sockets
I had the OT sitting on top of the chassis for the picture.I think there was plans to resue things that got shelved 1/2 way through. The OT seems to test okay
I get 250 between the red wires and around 53 between the yellows
Are your filament wires great thick solid copper wire? ( like house wiring) cause these are, right out of the PT insulated with a clear plastic

i see that. red, blue, & brown primary and yellow & black secondary.


my PT has green wires for the filaments: looks like 14AWG. the others are red - red, yellow - yellow and black - black for primary.


hack the PS board: there's -35V for bias available. better yet reverse engineer it.         

[PRR]

8417 was invented for an over-the-top Fisher hi-fi amp.

It is possibly based on 6550 but fudged for higher gain.

It got conscripted for many other things.

My impression, from running these in days when 8417 was in-stock at local TV shops, is that 8417 was fine for hi-fi duty, and much PA (I ran a dozen of them), but could be fragile in HARD work. And modern rumor is that one brand is extra-fragile, and neither brand has been made in decades.

Back-tracking to 6550/KT88 is for-sure the way to go today.

However 6550/KT88 will need TWICE the bias (and drive) of 8417.

As Pete sez, while 8417 may bias at -15V (for 560 Vp 300Vg2), 6550 in similar conditions will need -32.5V. More than twice! And as he says, there is a nominal -35V at C51. That looks fine, but leaves very little room for 6550 tolerance.

Change R30 22K (to C51) to 1K. Change R31 22K (to ground) to 47K. This gives nominal -34V to -23V. Be *sure* you smoke-test with bias turned all the way to -34V.

Break cathode leads and install 1 Ohm cathode test resistors.

Put DC volt-meter in mV and poke the 1 Ohm. Book value is 100mA per pair so 50mA per tube, which reads 50mV in 1 Ohm. This is 30 Watts idle dissipation. 70mA puts idle heat right AT the MAX rating of the tube, you need to stay below that.

You also need twice the drive. The more I look at that too-clever driver, the less I like it. Offhand I would steal a 5F6a Bassman driver. It will do fine fed with the +300V to screens.

If it really still has C50 line to chassis, clip it out and destroy it. Install 3-pin plug with green/yellow hard-wired to chassis. This aint 1972 anymore, and good thing too.

http://www.mif.pg.gda.pl/homepages/frank/sheets/168/8/8417.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/6550.pdf

[Toxophilite]

Cool! and thanks a lot!


Here's an idea I'll through out there, perhaps unrealistic
This unit is a little too much amp for me, If I want that much poop I'll use my super reverb
I was thinking that maybe it would make a good practice/low level bass amp, especially with that Big Iron


https://www.youtube.com/watch?v=999RqGZatPs


I was wondering about the possibility of moving the OT to over by the PT (kind of like the picture I posted, (though it's sitting on the 12at7 socket) and moving the speaker outs to the back
and then putting a 12ax7 preamp in the corner where the speaker jacks currently are
It's a little tight but maybe could be done
The inside would be gutted and rebuilt and the 8 pin sockets would be reused and 2 new 9 pins would be installed


Do you think this is too ambitious and prone to noise, given the size of the transformers and the fields they would generate?

[DummyLoad]

i'd leave it as-is (repaired, of course) and drive it with a pre-amp. something like an alembic F2 or a clone of one. measure the chassis. i may have a used front panel that will fit. you can have it for postage and six-pack. replace the PCB with a turret/eyelet board.


--pete

[Toxophilite]

ooo nice! a fancy schematic!
Thanks for taking the time!

Sadly though I can usually follow one there are some areas that I couldn't tell you if there was an error
The obvious stuff yes.


One thing I don't quite understand and it could be my reading of the schematic is the way the rectifier circuit works
It could be because I'm used to seeing PT designed specifically for tube rectification (guessing)
I know what a bridge rectifier circuit looks like and I use SS rectifiers on most of the amps I built
That looks like half of a 4 diode bridge rectifier?


In fact the whole filter area is unfamiliar to me but looks pretty skookum, I could probably build it. but understand it hmmm
That's a standby hooking the B Node to ground via the 47k 4W resistor right?


Curious as to the double throw power switch and the pilot light prior to the PT, Is that a personal preference?

[Toxophilite]

ON another note is it okay to clean up transformers with surface rust, Is there something good to paint them with..or should one even repaint them?

[sluckey]

May want to proofread that bias supply.   

[DummyLoad]

May want to proofread that bias supply.   

fixted. thanks... n00b error. sheesh!~


--pete

[eleventeen]

"I know what a bridge rectifier circuit looks like and I use SS rectifiers on most of the amps I built....That looks like half of a 4 diode bridge rectifier?"

Textbook voltage doubler. Used in lots of Bogens. Gives "twice" (actually a little more) than a Fender style "two legged" (I don't know what the proper name is) full wave recto. (A bridge rectifier is also a FW rectifier) But it also offers half the current of same. That's why you have the monster power tranny.

Note carefully how if you are going to create a bias tap by "stealing" one leg of the HVAC feed to a voltage doubler, you have to have that blocking capacitor before the "backwards" diode. Usually that cap is on the fat side, at least .1 and often .22 uf.

Google "voltage doubler" in Google images.

[Willabe]

I think it's a half wave doubler as apposed to a full wave doubler.

[DummyLoad]

I think it's a half wave doubler as apposed to a full wave doubler.

full wave doubler. aka, delon bridge voltage doubler.


--pete

[PRR]

> fixted

I don't understand C7 C8 C9 C10 and their balancing resistors?

The PT CT gives a "hard" half-voltage. Tie this over to C9 C10 and they will split-equal.

I guess you do need bleeder resistors because of the unexpectably high voltage (100V-200V more than we find in many-many guitar amps).

Agree that your doubler-bias for-sure gives ample grid range. I was clinging to the half-wave for simplicity (and less to go wrong), but it was right at the edge.

Not sure C16 is essential.

And, FWIW, my 150W/ch Bogens did not choke-filter the screen supply and I always thought they were quiet (this with PA speakers measured smooth to 50Hz). Of course guitar experimenters have reported "choke bounce" as a good thing.

R1 does not look essential. C1 "should" not be essential if the preamp is polite, does not leak DC. Even a little DC is tolerable if VR1 is not adjusted during the show. And depending on preamp, R1 C1 VR1 may all be redundant.

And yes, the doubler rectifiers are Full Wave on the AC side and 120Hz ripple on the DC side. They are just as good as a bridge (less diode drop) and slightly better than a CT 2-diode ("full wave") affair.

[Toxophilite]

I could be wrong about this but there doesn't seem to be a center tap on the PT
there's the 2 blk AC ins 2 heater wires 2 yellows and 2 reds total

[DummyLoad]

I could be wrong about this but there doesn't seem to be a center tap on the PT
there's the 2 blk AC ins 2 heater wires 2 yellows and 2 reds total

there isn't. none of the windings are center tapped.

--pete

[Toxophilite]

I didn't think so , but PRR seemed to , so I thought I'd point that out

[DummyLoad]

   - there are no CT on any winding. look at pics in opening post + i looked at part i have.

the B+ winding is 220V 500mA coupled to a delon voltage doubler. c9-c10 are stacks for plate supply. there is no connection at L1, c9 & c10.

c9-c10 are there b/c there is 600V at node. i don't like running caps at or > their rating(s). 2 x 350V caps and 40 cents in resistors are cheap insurance: 2 x 100uF 350V quality caps are about same cost as 1 50uF 600V cap. 2 x 100uF 350V 105degC part cost $3.50; one 50uF 600V 105degC cap cost $8.50. so 4 bux w/ R's vs. 8 & half bux. you do lose another couple of bux for board space and turrets/eyelets: overall still less cost.

agree, c16 is not essential: delon doubler bridge PS like more filter than FWB. bias supply is source for hum and the cleaner the better, so 75 cent part is negligible expense.

--pete

[Paul1453]

When you said the old owner claimed it was a guitar amp that used some kind of 7 tubes in it, it made me think of the Ampeg amps using 7027s with nearly 600V B+. 
I thought that would explain the heavy metal PT.  The Ampeg 7027 amps seemed like it would have used monster transformers like you have there.

The Ampegs were fresh in my mind from my line by line exploration of the Massive schematic library available here.
I haven't even gotten out of the A's yet.   

I finally made it to the B's tonight, only 24 more letters to go.   

[Toxophilite]

That's what he said initially. he later emailed and said he found the original tubes and that they were 8417s, then sadly dropped one, however they're expensive and not readily available so no big loss.
7027s were one of my thoughts too when I was trying to research the amp.

[PRR]

> I didn't think so , but PRR seemed to

Wasn't me.

There are several ways to do windings rectifiers and caps. I think I covered about three very quickly. Sorry if it was not clear.

Using my eeePC 701..... dang these keys are still tiny-tiny, plus I think my fingers are getting fatter.

[Toxophilite]

Personally I totally appreciated your input, and any and all input I get in my postings
I must have misunderstood what you meant
I assumed that perhaps as you didn't have the actual transformer in front of you it wouldn't be obvious that it didn't have a center tap.
My apologies for the assumption

You said this:
The PT CT gives a "hard" half-voltage. Tie this over to C9 C10 and they will split-equal.

[PRR]

> You said this:

You are correct. This is phrased so badly that it came out wrong.

As a huge fan of the doubler, I "think" (if that is the right word) of the non-diode end of the one winding as the total supply's "CT". This is all in my head.

Probably time to throw a brick through the computer and go to bed.

[Willabe]

     Good night our friend and as always, thank you, may your dreams come true. 

[Toxophilite]

Probably time to throw a brick through the computer and go to bed.




Well said!
I think it's almost always that time
Goodnight

[shooter]

time to throw a brick through the computer

thanks for Keeping service guys in business