7355/6L6 circuit

[Jack1962]

I posted on the schematics board about this I guess I should have just got to the heart of the matter , the 1963 Twin Reverb when it first came out had 7355 output tubes ,they where changed to 6L6. what I need to know is , is the circuit used the same circuit just adapted for the different tube pinout(and transformers) or did fender change the output circuit entirely?

[HotBluePlates]

I've never seen or heard of a 7355-equipped Twin. Do you have pictures of one, or a Fender schematic to show that?

The 7355 (which I'd also never heard of) has an 18w plate dissipation rating, below that of even 5881's. It seems improbable Fender would have ever used it in their highest-power amp (which as far as I know went from 5881's in the last tweed version to the 6L6GC in the first blonde Twin amp).

I seem to be finding mention of 7591-equipped Twin Reverbs (another tube in the 19w range), but even that seems to be a rare bird. Either tube would be a big step backwards in volume level, as you couldn't hope for more than about 60% of the 72w of total dissipation of the tubes, or about 43w of output. A Super of the era would be as loud/louder with half the tubes.

So same circuit? It's probably similar, but less current demand from the PT and less power throughput expected of the OT. Either would/could be smaller than the stock units.

[sluckey]

Google turns up some interesting info for the phrase "Fender Twin Reverbs 7355 power tubes". They are really out there. Several old ebay listings. Showmans too. I didn't see any schematics or pics though. I did find a tube chart...

[HotBluePlates]

Yeah, I saw the same on tube charts, but more info on the 7591 Twin Reverb (although even that is almost a unicorn).

[DummyLoad]

interesting. didn't know fender strayed away from the more conventional 6L6/6V6. datasheets say 40W is possible w/ 1 pair @ 400V B+ into a 5KΩ load. so 80W w/ 2 pair into 2.5KΩ, i doubt leo changed iron, twin OT is 2KΩ pri. so you push little 7355 a bit harder into 2KΩ load and get 5W or so more. 80W or 100W; still a VERY loud amp. would be cool to be able to hear/play one.

--DL

[PRR]

I suspect the 7355 is a slightly small 6L6GC, aimed at the Hi-Fi market. It may have been cheaper, intoductory pricing or after it failed to catch-on in Hi-Fi. I doubt the actual amp-build was any different.

7591 is a 6L6GC type frame, smaller, with much higher Mu(g2) for better sensitivity and less driver distortion and more NFB in Hi-Fi design. It does need different biasing, may want NFB tweak, and IMHO the old bottles were not as robust as a real 6L6GC. (Today's jars may BE 6L6GC parts with different G1 pitch.) There was some aggressive pricing on 7591, it became somewhat common, but at the very tail of mass tube production, so there was only small warehouse inventory and then some Mac fans bought-up all of that.

[SoundmasterG]

I suspect the 7355 is a slightly small 6L6GC, aimed at the Hi-Fi market. It may have been cheaper, intoductory pricing or after it failed to catch-on in Hi-Fi. I doubt the actual amp-build was any different.

7591 is a 6L6GC type frame, smaller, with much higher Mu(g2) for better sensitivity and less driver distortion and more NFB in Hi-Fi design. It does need different biasing, may want NFB tweak, and IMHO the old bottles were not as robust as a real 6L6GC. (Today's jars may BE 6L6GC parts with different G1 pitch.) There was some aggressive pricing on 7591, it became somewhat common, but at the very tail of mass tube production, so there was only small warehouse inventory and then some Mac fans bought-up all of that.

I've got a pair of 7355's at home that came in a little tube powered record player. They look about the same as a 7591. I would guess they were yet another RCA sourced and designed tube that was just a little different than their other offerings and targeted as being special in some way when it really wasn't much different than the others. Either that or it was a competitor's answer to a 7591. Their specs are very close to each other, but less than a 6L6GC. I was intending to use them in an amp project that will use the iron from that record player, and then when they die, I would going to rewire the sockets for 7591's and call it good.

I heard that after RCA stopped making tubes, they destroyed a warehouse of 7591's since at the time they were a super cheap tube, then later the price sky rocketed because they were so few and far between. The new JJ's are the same size bottle and supposed to be the same specs, though I haven't used them yet myself. The EH 7591's are a larger bottle but supposed to have the same specs as vintage ones...again I haven't used them. The EH 7868 sounds every bit as good as vintage 7868's and so far holds up nicely after over a year in weekly use in an amp build. The 7868 is supposed to be electrically the same as the 7591 so maybe EH used the guts in their 7591 and 7868...but that doesn't explain the overlarge bottle for their 7591 offering.

Greg

[DummyLoad]

7591/7868/6GM5 same tube - only difference is the socket.

--DL

[Jack1962]

Ok thanks for the info guy's , looks like it might be best to just do a retrofit of the GE Stereo Amp that they are already in , I checked it out , sound a lot like a Fender now lol  

[gldtp99]

here's the tube chart in my Blonde AA763 BF Showman w/7355's:


A couple of pics showing the output tubes:



Amp has unique PT and OT-----i can get p/n's later if anybody cares----- other than the 7355's, PT, and OT it is an AA763 Fender circuit.
Every BF AB763 Showman (w/6l6GC) I've owned has sounded stronger, deeper, and richer than this 7355 amp---- i have a Blonde AA763 Bandmaster (w/6L6GC) that is close to the performance to the 7355 Showman---yes, I've tried different 7355's.
An oddball Fender amp----- probably discontinued because they didn't sound as good in real life as they would have seemed to sound by the 7355 specs------my opinion only on this..........................gldtp99

[DummyLoad]

here's the tube chart in my Blonde AA763 BF Showman w/7355's:


A couple of pics showing the output tubes:



Amp has unique PT and OT-----i can get p/n's later if anybody cares----- other than the 7355's, PT, and OT it is an AA763 Fender circuit.
Every BF AB763 Showman (w/6l6GC) I've owned has sounded stronger, deeper, and richer than this 7355 amp---- i have a Blonde AA763 Bandmaster (w/6L6GC) that is close to the performance to the 7355 Showman---yes, I've tried different 7355's.
An oddball Fender amp----- probably discontinued because they didn't sound as good in real life as they would have seemed to sound by the 7355 specs------my opinion only on this..........................gldtp99

would you kindly consider to post the transformer part numbers and date codes?

the OT and PT look to be the same sizes as the 6L6 variant.

--DL

[Jack1962]

here's the tube chart in my Blonde AA763 BF Showman w/7355's:


A couple of pics showing the output tubes:



Amp has unique PT and OT-----i can get p/n's later if anybody cares----- other than the 7355's, PT, and OT it is an AA763 Fender circuit.
Every BF AB763 Showman (w/6l6GC) I've owned has sounded stronger, deeper, and richer than this 7355 amp---- i have a Blonde AA763 Bandmaster (w/6L6GC) that is close to the performance to the 7355 Showman---yes, I've tried different 7355's.
An oddball Fender amp----- probably discontinued because they didn't sound as good in real life as they would have seemed to sound by the 7355 specs------my opinion only on this..........................gldtp99

Cool , Thanks , so the circuit is basically the AA763 , with the exception of the transformers ?

[HotBluePlates]

Yeah, the info I'd found was that the 7591 and 7355 amps used the same power and output transformer, and different from any other Fender model.

PT - 125P19A
OT - 125A18A

interesting. didn't know fender strayed away from the more conventional 6L6/6V6. datasheets say 40W is possible w/ 1 pair @ 400V B+ into a 5KΩ load. so 80W w/ 2 pair into 2.5KΩ...

You're right, so G.E.'s numbers don't add up. They show 40w for 2 tubes in class AB, but 18w dissipation max for one tube (likely due to the small size of the bottle).

So they get 111% efficiency? Obviously either the dissipation numbers are too conservative or the operating condition is overhyped.

[gldtp99]

i pulled the 7355 Showman out, cracked it open, and took iron pics:
PT: 125P32B/606337


OT: 125A18A/606307


This is an Export amp with 117/200/210/220/230/240 VAC PT primary taps--so i don't know what the p/n would be for the non-Export PT.
I got the amp with the orig JBL 2x15 cab (4 ohm total load)---so the OT apparantly is designed to push a 4 ohm load.
I've seen another similar Showman head on ebay that was converted to 6L6GC x 4---- orig OT was present but PT had been changed to stock BF Showman/Twin Reverb 125P34A----- chassis hole had been enlarged to fit the physically larger 125P34A.
My 125P32B PT is 4-3/16"x 3-1/2"---- the 125P34A PT is 4-1/2" x 3-3/4" (outer stack measurements)--i knew the 7355 PT was smaller than the Showman/Twin PT the first time i reached inside to get at the chassis nut to take it apart.
Amp with matching 2x15 cab:


The choke is the standard 125C1A with a 606307 date code-- notice that there is no Mid pot on either channel, unlike later Showman amps that had a Mid pot on Vib chanel.........................gldtp99

[DummyLoad]

great info - thank you so much!   

--DL

[DummyLoad]

either the dissipation numbers are too conservative or the operating condition is overhyped.

i'm thinking maybe both at this point. it doesn't add up. i'm thinking of it this way, in AB peak power can be had exceeding the plate dissipation rating in short intervals - music is rarely average power so the question i think is; how long can you deliver 40W continuous sine power for with those conditions? do the claimed distortion specs seem exaggerated to you as well? 

--DL

[DummyLoad]

looks like a pair of JBL D130F in the cabinet - nice!!

very nice rig gldtp99.

--DL

[Jack1962]

I believe Fender said it was 72 watts , IMHO thats pushing those tubes just a little to hard .

[gldtp99]

Yes, this is an interesting oddball and a rare amp---as far as being a great player it does leave something to be desired compared to 6L6GC x 4 Showman amps---and 7355 tubes are rare items these days.
But one of the former owners seems to have used it as a working amp for some time judging by the condition of the tolex and grille cloth.
So it isn't a terrible sounding amp but my best BF AB763 and Blonde Brownface 6G7A 6L6GC x 2 Bandmasters give it a run for it's money as far as real life amp performance.
Thanks for giving me an excuse to pull the head out of storage and show it off----and I did re-glue some tolex edges while it's apart----- glue is drying now........................gldtp99

[HotBluePlates]

... in AB peak power can be had exceeding the plate dissipation rating ...

Instantaneous power dissipation can easily be higher than the plate dissipation rating, even in class A. This is equivalent to the moment the load line is higher than the plate dissipation curve. Overall, the assumption is this is a small percentage of the total duty cycle of the tube, and so doesn't materially overheat the tube.

But dissipation does not equal power output to the speaker. It's impossible to get 111% efficiency, and even class B2 is only gonna give you something in the neighborhood of 70%+ efficiency.

Wild theory with no proof: I bet Fender was sold on the 7355 based on the number you saw, and the cheap prices offered for the new tube. But just like the 7591 may be 6L6GC guts, but with a 19w rating, I bet the 7355 really needed a lower dissipation rating than the conditions suggest because of its smaller envelope. I'm thinking the protoype tube was capable of more power output, but the smaller bulb surface results in poorer transfer of heat to the air and necessity of reducing the plate dissipation rating. What if Fender had a large percentage of early failures, prompting a fast switch back to known-plan 6L6GC?

All that is speculation.

[DummyLoad]

But dissipation does not equal power output to the speaker.

i know that. case in point. 6550 Pdiss max is 42W - two tubes 84W, but 100W or more into a load is easily attained. in AB1/AB2 and B what you can safely drive into the tube as a maximum signal plate input power under a given load is relevant to the output power. another case in point - 6146 has a 20W max plate dissipation in CCS, so max diss. for 2 tubes is 40W in CCS, however, 2 tubes can deliver up to 80W into a 7K load in CCS condition.

--DL 

[SoundmasterG]

7591/7868/6GM5 same tube - only difference is the socket.

--DL

Yes, but that is with vintage examples. For the new ones, your guess is as good as mine since manufacturers like New Sensor don't necessarily share data of that sort. New Sensor's EH 7868 appears to be every bit as good as vintage 7868's, but their EH7591 is in a larger bottle than vintage 7591's, and there is really no reason for that since if the internals were the same then they should be able to use the internals of the 7868 in an appropriately sized 7591 bottle with the correct pinout...but they don't. JJ's 7591 has the correct bottle and pinout, but since I haven't tried it I can't comment on how it sounds or holds up in comparison to vintage 7591's.

I think the 7355 was GE's answer the RCA's 7591, but I would have to look through my Vacuum Tube Valley mags again...I think it was in there somewhere.

As for power output....my Bogen CHB100 that I modified made 54 watts RMS before I installed power scaling, which dropped the power a bit, though I haven't measured it yet. That is with four 7868's at 480v plate and 440v screen and a 5k primary impedance. That should make more like the 44 watts RMS that is in the RCA tube manual with those voltages, but it doesn't. I would guess it is because of the smallish output transformer myself, but perhaps the power output numbers are a bit optimistic too.

Greg

[DummyLoad]

Yes, but that is with vintage examples. For the new ones, your guess is as good as mine...

understood. i don't use new production tubes, so i must rely on feedback from those that do. thank you for the info.

I think the 7355 was GE's answer the RCA's 7591

7591 was designed and made by westinghouse and any RCA branded 7591s are westinghouse made. RCA & GE were same co. so 7355 was likely RCA/GE answer to the 7591.

my Bogen CHB100 that I modified made 54 watts RMS before...

hmmm IIRC, that amp used a quad of 7868 - not exactly stellar performance for a quad. I thought that was a 100W rated amp?

--DL

[clyde]

What is the OPT primary impedance of the Showman with the 7355's?  Same as 6L6GC?

[PRR]

> 40w for 2 tubes in class AB, but 18w dissipation max .... So they get 111% efficiency?

Dissipation is NOT output.

A light-switch outputs over 1,000 Watts but dissipates much less than 1 Watt.

Tube is not a switch; moreover in Audio we use Sine waves and the amp has to waste-off the non-Sine excess.

The best possible sine efficiency of Class B is something like 78%. That is: 100W from power supply, 78W to load, 22W lost in tubes.

Or: the output power "can" be 78/22= 3.5 times the dissipation.

2*18W= 32W dissipation "can" support 127 Watts of output.

FWIW, transistor amps often do 100W out on 50W dissipation, 66% efficient. By cheating the protection and pushing the THD you can get show-off numbers over 75%. (But put it on a crazy stage, you need all the protection and dissipation you can sell.)

Switch-mode amps do much better, by not even taking the power implied by the difference between Sine and supply. 100% on paper, 90% in practice. 100W out on 10W of heatsink.

Tubes don't come close. Triodes suck less when they try to suck more. Pentodes are better, but for G2 similar to Plate voltage you can rarely pull plate down to 10% of supply, and 20% is not atypical.

Your link to 7355 data shows actual output and dissipation on Page 8. Bottom (400V) chart. For 62V grid signal, the output is 42 Watts, the dissipation is dropping to 15W per tube or 30W total. 42W/(42W+30W) is 58% efficiency, a good but not fantastic number for medium tube amps. If pushed to 65V grid it's 45W out for 28W dissipation, 61% efficiency (but THD is about to rise fast).

813 has a condition for 67% plate circuit efficiency.

4CW10,000A has a condition for 49,950W in, 31,900 out, 63% efficiency.

6L6GC bonus condition (450V) is 94.5W in, 55W out, 58% efficient. Tung-Sol suggestion, last page, shows for 76V grid, 63W out, 19W per plate, 63W/(63W+38W), 62%.

Note in all these curves, Pdiss is actually dropping at Full power. Max diss is at part power.

In theory, the silent zero-output condition could be biased dead-cold. But that means more drive and also low-level distortion. For medium wall-power amps, there's little point in cold idle. But note the huge 4CW10,000A is suggested to idle at 3,700W, teeny compared to the 18,050 dissipated at full power. If they set idle diss similar to half-power diss, like we see on 7355 or 6L6GC curves, the 18KW idle is $3 an hour on the electric bill, $72/day, $1,440 a month (plus the fan/pump motor to move-out enough heat for three Maine houses). A couple 6L6 idle at a penny an hour.

[HotBluePlates]

Yeah, I wound up realizing all this when looking more at the sheets and thinking about power input vs power delivered to the load.

[SoundmasterG]

Yes, but that is with vintage examples. For the new ones, your guess is as good as mine...

understood. i don't use new production tubes, so i must rely on feedback from those that do. thank you for the info.

I think the 7355 was GE's answer the RCA's 7591

7591 was designed and made by westinghouse and any RCA branded 7591s are westinghouse made. RCA & GE were same co. so 7355 was likely RCA/GE answer to the 7591.

my Bogen CHB100 that I modified made 54 watts RMS before...

hmmm IIRC, that amp used a quad of 7868 - not exactly stellar performance for a quad. I thought that was a 100W rated amp?

--DL

I had to look up the VTV article again on the history. 7591 was Westinghoues...thanks for correcting me on that. 7868 was RCA's answer, and 6GM5 was Sylvania's answer. The 7355 looks very similar to 7591 with a bit lower max ratings and slightly lower power. Also looks like it was a lower gain tube and needed a bit more negative grid bias.

Bogen was pretty optimistic in their ratings....they call the amp a CHB100 but I think that is probably peak power...as when I measured that, I was getting 97 watts. They have the impedance correct to get good power in 5000 ohms plate to plate, but the transformer is small....If I get the right combination in the future of transformers and tubes I could test with a better setup and see what kind of power I could get out of a quad or pair, but I would guess the 88 watts for a quad or 44 watts for a pair that RCA quotes was probably a bit optimistic. Probably more like 35 watts for a pair is more realistic, but who knows without testing. I haven't measured the amp since I put power scaling in, but since the addition of that dropped plate voltage from 480v to ~460v, then overall power output probably dropped a little from the 54 watts RMS I measured previously. It sounds good and does the job so that is what counts!

Greg