1969 Deluxe Reverb biasing question

[tonyboy]

I just recently bought a 69 DR. I am not an amp tech, and I have an amp head bias probe and plate voltage probe. I have some specs on what occurs in the biasing that I did and some questions to any techs that can shed a little light on what i've done or didn't do. I have an old GE 6V6 and a used new issue tungsol 6v6 they both biased at 21 and 22 ma. The voltage probe shows a measurement of 401. the zamp sounds fine and I did not use the voltage probe on both tubes, should I is there inconsistencies in plate voltage separately for each tube. The rectifier is a JJ GZ34 and I have two rectifiers one in a 69' Vibrolux Reverb and one in a 65' Super Reverb that are Mullard GZ34. Should i try those to see if the voltages would be more or less than the JJ's?

[telex76]

Numbers look fine where they are.
Won't hurt to check out the other GZ34's if you want to, but you probably won't get any closer to correct plate voltage or correct bias than you are now.

[SoK66]

Pretty sure that amp calls for a 5U4GB rectifier, but if the plate voltage is at 400-ish you're OK. If the tubes bias that closely then the p;late voltage sould be very close as well. I'd measure it to be sure.

Are those 6V6GTs or 6V6GTAs? The former are 12 watt tubes, the latter are 14 watters. Yoy can bias the latetr a bit hotter, but depending upon plate voltage, 21-22 is a bit high.

[Wally]

I am taken aback at the plate voltage....401V is rather low for these days with a GZ34 in that SFDR? NO problem...but it makes one wonder. IN the summer with higher demand for electricity, our wall voltage goes down. I observed a 12VAC swing last summer while I was biasing an amp...125AC in the morning to 113VAC in the heat of the afternoon. ONe would want to take note of the wall voltage.
I would suggest putting the amp back on the bench, noting the wall voltage, and then checking all voltages in the amp along with the current draw. I also have to ask if this amp has been recapped? Why...because 401V is slightly unusual adn I like to understand unusual situations...why are they that wawy. Ex: on another forum...a member was curious about this currentd raw and plate voltae relationship. His '76 SFDR with the (called for on the tube chart) 5U4, which drops more voltage than a GZ34 should, had 460V on the plates. The difference between 460 and 401 makes one wonder. The schematic calls for 415, IIRC....and that would be at something closer to 110-117VAC at the wall.

[tonyboy]

The fuse is a 2 amp slo blo, would a 5UG have a 3 amp fuse? I will try the other rectifiers. the amp sounds great by the way.

[Wally]

2 amp slo-blo with the 5U4, also. Any other rectifier is going to lower your B+. This is why I posted my first response about the voltage. Imho, it is a necessity to know your wall voltage. Why? Well.....if your wall votlage is at 122VAC and your B+ is at 401V, then that amp is not functioning like any other BF/SF DR that I have seen. Also, if the wall voltage is varying due to demand on the power supply grid and you set the amp up 'hot' with lower voltages, the amp and tubes might not like what goes on with higher voltages that could occur at another time.
That low B+ is also why I asked about the electrolytic capacitors. Old e-caps do not yield voltages in the manner that they should. 'Sounds great".....with a low B+ like that, I would expect the amp to have a 'browner' sound than oen would expect form a DR that is showing 'correct' voltages. One may or may not like that......but imho, that voltage needs to be understood. I am not saying that anything is wrong but just that I would have to come to an understanding of why the B+ is that low.

[tonyboy]

Wally, I measured the amp with the bias and voltage probe once again and the bias for the tubes is consistent with all 3 rectifiers 415-417volts and 20-21ma on the tubes.

[Wally]

Okay, tony. So, with GZ34's the amp is running at cheamtic typeof votlages......at whatever wall voltage you have there. We still don't know that. ????HOw did the B+ change from 401 to 415-417V??? Anyway, you are dissipating what......soemwhere between 63.88% for a 14 watt 6V6 to 74.53% fro a 12 watt tube, right? So, one might really want to know...as SOK66 queried....what GE 6V6 tube you have in there....adn once again....what is the wall votlage. BEcause...if that is a 12 watt GE 6V6 and if the wall voltage is low right now, then thnigs might get 'reddish' o n those plates if the votlage that the amp sees is higher at the wall.
Thsi will be the last tiem I say this...but that wall votlage is of interest. IF the wall votlage is 'normal'...around 120VAC or so for modern times, then I am still curious about other things in the amp because even with a 5U4, that B+ is a bit uncommon these days. WIth the higher yielding GZ34, I consider it definitely low. AS I said earlier, I am not saying anything is wrong; but that B+ makes me curious...about the PT...about the e-caps....about the wall voltage....about the earlier 401V on the plates...... Inquiring minds and all.

[tonyboy]

Ok, I turned the multimeter to 200v and tested the power strip that I plugged my amp into and it read 121.1. I think that the 401 voltage was at a higher ma bias when I decrease ma voltage goes up. As it stands all the rectifiers put out 415 to 417 VDC and the tubes are at 21-22ma.

[Wally]

Okay....so if that GE 6V6 is a 14watt tube, you are fine. IF it is a 12 watt tube, you are treading water with sharks close by, imho, as SOK66 noted.

with 417V at 21ma.....


GE6v6GTA---14 watts max dissipation
.021 x 417 = 8.757 watts of plate dissipation
8.757/14 = 62.55% of max plate dissipation

GE6V6GT...12 watt tube....
8.575/12 = 71.46%....just a bit high for comfort.

IF that GE 6V6 is a GT and it is showing 415V....then...
.02 x 415 = 8.3/12 = 69.16%...jsut under the 70% figure for 'safe' plate dissipation.

It is important to know what voltage is with what tube and what the specs are for that particular tube.

Good luck with it.

[tonyboy]

I do have a 5U4GB rectifier, if I used that what should i see for PV and what ma should I bias? As it stands, I lowered the ma to 18 for both 6V6's they match nicely and the PV is 417. What PV will a 5U4gb give me? Where shoud i bias the 6V6's?

[Wally]

tony, the 5U4 will drop more voltage...so you will see a lower B+. Plug it in and see what you get. YEs, you will need to rebias with the new voltage.
I have one thought, though....adn it is related to this lower voltage. IF htis PT is not original andif it is designed to carry a GZ34 rectifier instead of the GZ34, you might not want to run a 5U4 in the amp. Teh 5U4 draws more heater filament current than does the GZ34. A DR with a PT that is designed to run a GZ34 tube will not allow the amp to perform properly with a 5U4 drawing it down. I have witnessed the results....adn they are at best a compromised sound at lower voluem. AT worst, the PT fails with prolonged use of the inappropriate rectifier. This is why I was stressing the importance of knowing this amp and why the voltages seem to be low. Imho, this amp needs to be seen by a knowledgable tech who will evaluate every aspect of the amp. I am a cautious person, and I like to know exactly what is under the hood.
OR....keep running the GZ34. A PT that is designed to handle a 5U4 can easily handle a GZ34 with the only concern being higher voltages....which doesn't seem to be a concern iin your amp, right? I see no need to change rectiifers because your voltages are spot on for a DR anyway. IT is simply odd that a SFDR has a B+ down that low while running a GZ34, ime. That said, those current draw readings put the amp in the 'safe' zone with regadr to plate disspation.

417V x .018ma = 7.506/12 = 62.5% of max plate dissipation...safe for the 12 watt tube...and even safer for a 14 watt tube.


AS you will discover, I am a persistent soul. Has this amp had fresh electrolytics installed in it?

[tonyboy]

I just removed and checked under the chassis, and under the cap can it has 5 blue 475VDC sprague atom electolytics. They look pristine. There are 3 of 4 (didn't count exactly) blue molded caps on the right side and a brown turd at the far right. Two wires to the bias pot. All trannies are from 69 ranging from early to mid 69 the PT is 32 week of 69. All Schumachers.

[nonvintage]

I remembered my old username and password. I don't usually use tonyboy.

[Wally]

nonvintage, waht color are those e-caps under the can? IF they are blue, then they are relatively 'fresh', I would think. FWIW, there are date codes on e-caps, usually. Adn...looks don't matter.....unless there are sugns of phsycial leakage. IF those caps are yellow/orange, then they are original and need to be changed, imho. The e-caps inside the chassis that are of concern are the bias caps and the bypass caps. Pics?

[nonvintage]

They're Sprague 16v 475 VDC they're blue and they look brand new. Wally, see post 13.