All ok ? Trannies specs,Bias, Power tubes specs

[espinete]

Hello !
An amp builder told me that my new amp measures 485v bias 39.40mA. My tester says more or less same thing.

Power tubes he installed :
Tung Sol reissue 5881
23w 400v

I called a technician because my pedals sound too gainy with the new amp. He asked for trannies specs . Transformers are custom. Specs below.

Transformers :

POWER TRANSFORMER
Lamination: 4 1/8" & concentration : 2"

Secundary :
365 V@170mA red
335@170mA red/orange
55V red/blue
CT red/yellow
335V red/orange
365V red
3,15V green @4A
CT green /yellow
3,15V green
5V yellow @3A
0V yellow
Installation 2 3/4
x 3 7/16


OUTPUT TRANSFORMER
Primary Impedance 4.2k
Lamination: 3 3/4", concentration: 1 1/4"
Installation : 3 1/4" x 2"
Dimensions 96mm x 79mm x 80mm

Secundary :
0 Ohm black
2 Ohm white
4 Ohm yellow/green
8 Ohm green


Is everything correct ?

[sluckey]

With 485V I would use 6L6GC rather than 5881.

[Stevem]

Your tech wants the impedance of the output transformer, but yes as posted above your running those 5881 tubes with too much plate voltage and in turn too darn hot.
So much too hot in fact the lowering the idle current setting will only kick up the plate voltage even more so yes you need to run 6L6GC tubes or 7027A tubes.

[espinete]

Thank you !


Are the preamp readings ok ?

[Stevem]

Your plate voltages on atleast two of your preamp tubes are way too high, but with that being said no one here on this board can help you any further without a schematic of the amp from the tech that built it , or some really really good internal pictures of the amp broken down into atleast 4 sections!

[pdf64]

Hello !
An amp builder told me that my new amp measures 485v bias 39.40mA. My tester says more or less same thing.

Power tubes he installed :
Tung Sol reissue 5881
23w 400v...

Previous posts from you note that your amp has a rectifier switch, giving solid state or GZ34 HT mode options; which mode was your amp in when the above measurements were made?

Also, as per my repeated queries on your various TGP threads regarding this amp, what is the intended and actual, measured mains Vac, and the actual, measured heater voltage (eg 6.3Vac), from your power transformer?

And which HT wires are used from the power transformer to either / both rectifiers, eg red or red/orange? A previous photo seems to show the red wires going to the silicon diode tag strip, but further detailed photos are needed to be sure how it's been done.

Or an accurate schematic; given the numerous deviations from an SR AB673, it may be seen to be the builder's duty to supply such info, and somewhat remiss of them not to do so.

It would be helpful to your goal for you to answer these queries, and the others that have been previously asked.
If there's something that you don't quite understand about any of them, please enquire further.

Thank you !


Are the preamp readings ok ?

As per my response to the same query on TGP, and just in case you choose to delete that thread (an action I perceive as ignorant, and ungrateful to other thread contributors ) too ...

You seem to have queried what you may think is a discrepancy on the typical voltages noted on Fender's info, ie that 460V is indicated for one V3 plate, 450V for the other.
You've misunderstood the drawing somehow, in fact both V3 plates are linked and are as being 450V. The 460V figure pertains to the HT supply to the reverb driver OT, whose primary feeds V3, ie there's a 10V drop across the reverb OT primary.

The most likely reason for V1 terminal 6 reading a little higher than expected is probably due to a 5751 being fitted in V2. The 2nd stage of both channels share a cathode resistor, at their pin 8s. A 5751 triode section tends to draw a little more current than 12AX7 triode, hence in such an arrangement, the 5751 forces the 12AX7 section to draw a bit less current than would otherwise be the case (by the same effect, the 5751 draws a little more current). It's not related to the normal channel being disconnected etc.

[sluckey]

You have expressed a concern about the B+ being 485V. And IMO, 485V is too high for 5881s. However, 485V is perfect for an AB763 Super Reverb using the proper tubes. The schematic shows 465V and the notice on the schematic says voltage values shown are +20%. Your measured 485V is only 4.3% higher than the 465V value shown on the schematic. This is well within Fender's +20% tolerance and is quite normal and acceptable.

Please refer to the highlights on the attached partial schematic for the Fender AB763 Super Reverb. I suggest you accept these perfectly normal voltages and put the proper tubes in the amp. Running 5881s in this amp will likely have short tube life and they may fail in such a way as to kill your output transformer too. Just not worth the risk IMO.

[espinete]

I deleted thread because there was a lot of mistakes from me and we didn't get any solution or conclusion.
But i promise don't do the same with current ones.

Hello ! I put a pair of TAD 6L6WGC-STR this morning. BIAS set around 42mA. More or less same readings at preamp tubes . Apparently, the amp is all right. But when you add any od pedal , it sounds like a pedal with a very old battery inside. At any volume. With four different pedals. I changed cables. I was thinking about my voodoo lab power 2. I bought a boss psa pedal transformer. Same results.

[espinete]

I came to the conclusion that it could be my imagination. Until I met the case of a friend with the same brand and model of amplifier.
It's a long way to the only good technician i know. And he's very pricey . So, suggestions are appreciated. And i'm not sure about he will find a solution. I want to try as many things as I can before i leave my amp with him

[espinete]

I need more time to check this out but with 6L6GC and turning the treble knob down i'm getting with od pedals very similar sound than i used to

[pdf64]

You have expressed a concern about the B+ being 485V. And IMO, 485V is too high for 5881s. However, 485V is perfect for an AB763 Super Reverb using the proper tubes. The schematic shows 465V and the notice on the schematic says voltage values shown are +20%. Your measured 485V is only 4.3% higher than the 465V value shown on the schematic. This is well within Fender's +20% tolerance and is quite normal and acceptable...

Very fair points, but the thing is that @espinete seems to have had this amp custom made to accommodate the 400V plate limit of said TSRI 5881,

I had a headstrong king reverb (heyboer transformers) and a 1966 super reverb (mercury magnetics transformers) with all carbon resistors, i believe. In both of them i disconnected normal channel, tremolo and NFB wire. I loved them because its clear sound and gain. Both had tung sol 5881 and a zener circuit to dispel extra voltage. Now i have a boutique amp with same specs, except the trannies. Hand made. Power tranny special made for these tung sol 5881 reissue tubes...

It seems reasonable to ask the builder 'what gives?'

Except that photos below and elsewhere seem to show that the lower HT red/orange PT wires seem to be connected to the tube rectifier socket, the higher HT PT red wires to the silicon rectifier tagstrip.
So it seems possible that the 484V plate is in the silicon rectification mode, and the GZ34 mode may elicit a more 5881 friendly HT; @espinete, it would be useful to clarify this point - pretty please?

But I may be going nuts, as the rectifier sockets seems wired up wrong; is the photo reversed or something?

Does the amp even work with the rectifier switch in the GZ34 mode?

So many questions that @espinete leaves unanswered
Sorry that I'm crabby today; I had a massive dose of steroid as part of a medical procedure

[pdf64]

I deleted thread because there was a lot of mistakes from me and we didn't get any solution or conclusion.
But i promise don't do the same with current ones...

Making mistakes is a crucial part of the learning process. That's why a group learning environment can be better than one to one; more mistakes can mean more learning opportunities for everyone in the group.
And a benefit of an internet forum is that group covers the whole world forever on
Some excellent, informative posts that knowledgeable folks to the time to write, which might have helped others, have disappeared; I had been planning to cite at least one.
Well, I guess the info in forums only lasts as the forum site gets funded - so much arcane knowledge has vanished with vintageamps.com/plexiboard going down ).

[pdf64]

...485V is too high for 5881s...

I put very little credence into any of the tube info provided by companies owned by New Sensor, eg what resource has been applied to verifying the bogie tube data they're publishing as being applicable to their product http://www.tungsol.com/specs/5881-tung-sol.pdf is a scan / scam but we have to be grateful that such even probable shysters as these are bothering to actually make tubes.
I think that the limits, characteristics and variance of New Sensor tubes, as with most other current production, is unknown, it's a sellers market, but perhaps the drive for that is that consumers would rather pay extra for gold plated pins than characterisation testing, even assuming that the competence and resource to do that hasn't died out.
The audio tube market was basically a parasite on the military industrial complex's demand for tubes. If folks were prepared to pay the true cost of good quailty, properly characterised tubes, ie without the subsidy provided by the military industrial complex, then the market would provide; as it is, golden age BS nonsense branding of 'high variance/unknown nominal' product is all the market will stand.

[espinete]

Thank you. I'm at work. I'll answer the best i could later by watching inside the amp. Do you want me to tell you all the rectifier wire connections?
No reverse pics, i believe, but a few were taken from the left side for a better perspective

[pdf64]

OK, thanks, I think about what best you can do. For the moment, one question per post; does the amp work in GZ34 mode?