B+ voltage too high on 5f1 champ

[frank90petrosino]

Dear All,

I'm building my first 5f1 champ tube amp and during the first voltage testing I performed too high B+ voltage. I used Tube Town tt-5e3-vpo power trasformer.

In order I'm measuring these values with no tubes:

242 Vac primary voltage;
342 Vac HT secondary of PT;
6.3  Vac across filaments and pilot light;

After that I pulled in the rectifier tube TAD 5y3GT/6087 and I discovered:

460 Vdc on B+

Could you give me some suggestions? It is the wrong rectifier tube for this transformer? Do you thing I need to order the TAD 5Y3WGTB type that have more "sag" like NOS tubes?

Thanks

[GibsonGM]

Hi, welcome to the forum.  Can you post the schematic you're working from?

Is this your transformer?  https://www.tube-town.net/ttstore/tt-powertransformer-fender-style-5e3-laydown.html

Haven't built one, so not sure of those voltages (unloaded).  Yes, it seems high, altho it will drop when it is loaded by going thru filtering and cathode resistor. Some net searching for build voltages will probably confirm this. 

I'm sure someone w/experience with a Champ will come along who has seen this (generally build 'em and then fire 'em up mostly complete). As long as it is spec'd right and you're in the ballpark, checking this is probably a good idea, but I wouldn't be getting worried just yet 

[anotherjim]

Should you get anything on B+ without the rectifier?
I would fit a 1M bleed-off resistor between the final B+ dropper (across C5) and ground. You may just have some funky residual voltage thing going on.

From this site...
https://robrobinette.com/How_Amps_Work.htm

Rectifier fitted and all tubes out, you could see around 450v on the output transformer B+ since it can charge the capacitors up to the peak of the secondary voltage which is measured in RMS AC (so multiply that by 1.4 and subtract the rectifier diode voltage drop). And as said above, offload secondary voltages go a little higher.

[frank90petrosino]

Thanks a lot for suggestions,

I working exactly from the RobRob schematics posted by Anotherjim and my trasformer is this one posted by GibsonGM https://www.tube-town.net/ttstore/tt-powertransformer-fender-style-5e3-laydown.html.

Some net searching for build voltages says around 370Vdc but thanks to Anotherjim explanation I realized that it is loaded voltages as it's showed on RoRob schematics.
I can not read anithing on B+ without the rectifier.
Unfortunately now I don't have 1M resistor 0.5W to try immediately.


If I understood good the Anotherjim explanation, the B+ I'm reading (460 Vdc) is quite normal? Could I put-in pre and final tubes (Genalex GoldLion 12ax7 and 6v6GT) and read voltages without damage them?

[mozz]

Read your b+ with all tubes installed and circuit completed.  Or tack a big ass 3k or 4k across the B+ with no output tubes and it will give you an idea of what you will be near.

[ThermionicScott]

After that I pulled in the rectifier tube TAD 5y3GT/6087 and I discovered:

460 Vdc on B+

Fear not, your amp is doing exactly what should be expected! 

As anotherjim pointed out, with no signal tubes installed, B+ will rise to about 1.4 times your secondary voltage rating.  That's why Leo Fender specified 450V filter caps in an amp that ran closer to 350V in service.

And once you have charged up those filter caps, they will remain at ~460V and slowly dwindle as capacitor imperfections discharge them (unless you do something to speed up the process.)  When you have signal tubes installed, they will bleed off most of the charge when you power down the amplifier.  Some people install high-value bleeder resistors to ensure that the caps have a way to discharge when the amp is off, but I usually don't bother.

[frank90petrosino]

Hi everyone,

thank you for your advices. Now with tubes and speaker in I read these values:

B+   401 Vdc
B+1 340 Vdc
B+2 290 Vdc

pin 3 (plate) of 6v6GT 388 Vdc

All other check points as reported here https://www.flickr.com/photos/97949719@N03/49595930948/in/photostream/
stay into a range of +/- 15%

My first impression playing a bit with the amp is it sounds very good but i'm still building the cabinet.

As ThermionicScott explayned I realized that with signal tubes installed, the capacitors bleed off very quickly.

What is your impression about these voltages? Do I really need to decrease them? In case I can order  and try a TAD 5Y3WGTB that should have the "SAG" like a NOS 5Y3, or you can suggest me something else?

Thanks

[ThermionicScott]

You are probably fine at this point.  The voltages are less of a concern than the plate dissipation.  What is the 6V6 cathode voltage?

I looked up your TAD 5Y3/6087 and it is indirectly-heated, so it will produce higher voltages than a "real" 5Y3.  I use regular 5Y3s, but as much as a stickler for the details as anything. 

[frank90petrosino]

I'm using a wirewound cathode resistor of 470 ohm 5W 5% and the voltage is 21.4 Vdc

If I'm not wrong I have 45.5mA bias current on chatode and assuming 95% on the plate I have 43mA * (388 - 21.4) Vdc = 15.7 W that is 112% of a 6v6GT.

What do you think?

[ThermionicScott]

I'm using a wirewound cathode resistor of 470 ohm 5W 5% and the voltage is 21.4 Vdc

If I'm not wrong I have 45.5mA bias current on chatode and assuming 95% on the plate I have 43mA * (388 - 21.4) Vdc = 15.7 W that is 112% of a 6v6GT.

What do you think?

Your math looks right to me.  Some 6V6s draw more current than others, so this tells me a lot more than just the supply voltages.  Lots of blackface/silverface Champs run at 16+ watts -- if you're using a good quality 6V6 (like old American/European production) and it isn't redplating, one option is just to call it good!

If it were my amp, though, I'd go ahead and buy a regular-style 5Y3 so it will behave more like a 5F1 was intended to.  The TAD 5Y3WGTB is probably good, eBay has tons of vintage 5Y3s that are used but test good as new.  I got about half a dozen from a seller years ago, so I'm probably set for life!

A real 5Y3 should knock down your B+ 20-30 volts and get your dissipation closer to 12-14 watts.   

[mozz]

+1
Champs mostly run right where you are now.

[frank90petrosino]

I'm using:

Genalex Gold Lion 6V6GT
Genalex Gold Lion 12ax7 balanced

I read about that tubes as the best 6v6GT and 12ax7 on the market. I wish my choose was good.

I'm looking for a regular-style 5Y3 and I will let you know about the new voltages I will have.

Thank you guys 

[iainpunk]

I'm using:

Genalex Gold Lion 6V6GT
Genalex Gold Lion 12ax7 balanced

I read about that tubes as the best 6v6GT and 12ax7 on the market. I wish my choose was good.

the differences between 12AX7's from different brands are so minute, that null tests have less than -60dB of output, which suggests that there is less than 0.1% difference between tubes from different brands and dates.
the difference between 6V6 tubes is in the gain, without affecting the frequency response much.

also, there are only 2 factories that make tubes commercially, all tubes come from those factories, despite the different brands that are put on them.

cheers

[ThermionicScott]

also, there are only 2 factories that make tubes commercially, all tubes come from those factories, despite the different brands that are put on them.

Well, 3:
JJ (Slovakia)
Reflektor (Russia)
Shuguang (China)

[iainpunk]

i didn't know about the china factory!
i'd love to learn to make my own tubes, i know of a Belgian (?) woman making her own tubes, but she doesn't sell commercially.
i'm almost sure i could make my own tube opamp that fits a DIP8 

cheers

[PRR]

The whole history of the Champ is about taking a very low price amplifier and over-volting it year by year to stay competitive with other beginner amps. Last I heard Fender was flogging 17 Watts on that 12W 6V6.

Just do it for now. When 6V6 replacement costs get truly painful, then re-think it.

[davent]

also, there are only 2 factories that make tubes commercially, all tubes come from those factories, despite the different brands that are put on them.

Well, 3:
JJ (Slovakia)
Reflektor (Russia)
Shuguang (China)

https://hackaday.com/2020/08/06/just-who-makes-tubes-these-days/

[frank90petrosino]

Thank you for these informations. So the different brands just select, re-label and double the price? 

[amptramp]

Someone at diystompboxes should buy up an old tube factory and start making what we need.  Somebody must have the vacuum pumps and the various bits of metal-bending machinery somewhere.  It wouldn't be that much more expensive than a semiconductor foundry.

[ThermionicScott]

Thank you for these informations. So the different brands just select, re-label and double the price? 

There are a couple things going on.  New Sensor (Mike Matthews's company) has bought up a bunch of classic brand names like Tung-Sol, Genalex, Mullard, etc.  So a lot of Russian tubes are going out under those labels with minor internal differences.  The other thing is tube audio related companies that slap their own label on other manufacturers' tubes, which has gone on forever in the tube world (yeah, Lowery didn't make the tubes you found in that old organ. )  So anything labelled Groove Tubes or Tube Amp Doctor is straight-up relabelling.  After some time reading and looking at pictures of the available tubes, it becomes pretty easy to identify who actually manufactured a particular tube.  And there are sometimes clues in the product name -- Groove Tubes often puts a "C" or "S" or "R" suffix on a tube number.