5F1 build problem

[alex29]

I have build a new 5F1 based on original schematic except from un additional choke (hammond 4h) from 5E1 and that, there isn't the negative feedback loop (22k resitor to ouput jack).

So, works fine until the volume turned up to 3 o'clock when it starts a squeel (like a very loud feedback). A litle bit before and after of this volume point there is a drop of volume. I have double and triple check the joints and the schematics.

have any idea ? thanks

[MWJB]

Can you supply us with DC voltages at 6V6 pins 3, 5 & 8 please? 12AX7 pins 1, 3, 6 & 8.

Does it still do this with the 12AX7 pulled out?

Some pictures may be useful.

The original Fendr schematics don't really have enough information to ensure a trouble free build.

[alex29]

Thanks a lot MWJB !

The squeel disappeared when 12AX7 pulled out.

My multimeter chose the occasion to dead...... I'll inform you when i have news.

[alex29]

Hi MWJB !

So, my PT is a fender universal 50W 350-0-350, 6.3, 5.0 from tube-town.
The voltages on both 2 red wires are 384ac (to ground). 7.4 ac and 5.9 ac are the filamments voltages.

On B+ is 470vdc

The voltages with tubes are :

6v6
pin 3 467vdc
pin 5 20vdc
pin 8 24vdc

12AX7
pin 1 265vdc
pin 3 2.1vdc
pin 6 259vdc
pin 8 2.1vdc

What's qoing wrong ? This PT is too big ? Is there PT fault ? Can i reduce the PT voltage ? or i have to buy new tranny ?

Thanks

[Bruce / Mission Amps]

Hi MWJB !

So, my PT is a fender universal 50W 350-0-350, 6.3, 5.0 from tube-town.
The voltages on both 2 red wires are 384ac (to ground). 7.4 ac and 5.9 ac are the filamments voltages.

On B+ is 470vdc

The voltages with tubes are :

6v6
pin 3 467vdc
pin 5 20vdc
pin 8 24vdc

12AX7
pin 1 265vdc
pin 3 2.1vdc
pin 6 259vdc
pin 8 2.1vdc

What's qoing wrong ? This PT is too big ? Is there PT fault ? Can i reduce the PT voltage ? or i have to buy new tranny ?

Thanks

The B+ plate voltage is outrageously too high on the single 6V6 power tube and you should have zero volts on lug 5 of the 6V6 socket.
You have a bad 6V6 tube or a miswired socket somehow.

First thing I would do is use the choke in front of the first filter cap and take all the B+ from after the choke.
Assuming you are using something around 500 ohms for the cathode resistor of the 6V6, the power tube is idling at around 21 watts!!! It
should be idling under 15 watts.
The only way to do that with the PT you have is to use a very soft rectifier and or, use the choke as mentioned above and turn the power supply circuit into a choke input filter.
This will give you about 340vdc to 350vdc on the plate of the power tube.
If you had a bigger OT you could use a 6L6 in this one.

[alex29]

Thanks Bruce !

I made your suggestions. I put the choke (hammond 159s 4H ) in front of first filter cap (in series) and the voltage reduced at 365vdc. it's ok i think. The extremelly loudness also reduced but it gave me a strong hisss... (maybe the wires are too long (?) - choke hasn't polarity - what else ?).
Can I use a bridge rectifier instead of choke in front of first cap ? (or before the 5y3gt !!!! ??????)

I'll check all voltages again. My OT is 125ESE.

Thank you very much !!

[Bruce / Mission Amps]

I'll check all voltages again. My OT is 125ESE.

Thank you very much !!

Remeasure it all again and repost the results.
If that 6V6 tube is not damaged from the high B+ and high idle current I'd be a bit surprised.
Look carefully at that voltage on lug 5 of the 6V6 socket.
With respect to ground, it should be almost zero.

[Enzo]

REmove the 6V6 and power up the amp. NOW measure the voltage at pin 5. If it is about zero, then the 6V6 was bad. If it is still +20v or whatever, then the amp is miswired as Bruce said or your coupling cap is bad.

First place I might check for wiring trouble is a grid resistor not grounded there.

[alex29]

Hi,

I changed the 6v6 and now i have few mv (I think its ok now. maybe a damaged 6v6.....). But the main problem remains. The voltage is too high (467-470 V). If i use the choke in front of B+ the voltage reduced at 365 V but a very strong hissss apeared (miswiring ?) and the strong sqeel at 3 o'clock remains.

Thanks Enzo, Thanks Bruce

[tubeswell]

Hi,

The voltage is too high (467-470 V).

Hmmm, maybe 360 at each end of the HT winding sounds as if it is too high. I'm doing a 5F2A build and had to get the power tranny built (because they are not available locally). The blokes who worked it out for me calculated 300 at each end to give me a Plate voltage of 335 (which I am curious about). But maybe that's part of your problem.

[alex29]

Thanks !

any idea to reduce the voltage ? (zeners ? bridge rectifier ?) the solution with choke seems to doesn't work.

Thanks

[tubeswell]

Try various different size high-power resistors (maybe?) on the plate line.

[MWJB]

The 365v b+ is not directly the cause of the problem. Most Champs run more than that without issues (I can run mine at 450v on the JJ6V6 plates, no problem). When you used the input choke did you put a filter cap at the upstream end? 365v with a 6V6 is just fine. The choke input is a better option than B+ dropping resistors.

Because you have used such large PT (nothing specifically wrong in doing so) a single 6V6 is probably not drawing enough current to pull down the B+ significantly, you could try a 5R4 rectifier, but again it wion't drop the full potential off of the B+ because of the relatively small current draw.

Alternatively, leave it as it is & switch to 6L6/KT66/EL34/6550 for the power tube and bias accordingly (OT is rated for 80mA/15W). A tube & cathode resistor (10W) costs less than a PT. Bypass cap at pin 8 should be 100v rated.

Your squeal is not directly related to the power supply/voltages. Start by connecting the NFB loop does this help. Do you have a photo of the wiring? Are your grid wires short & sheilded? Have you checked integrity of your grounds? Have you made sure that you don't have HV dc and grid wires running parallel & close?

[alex29]

Thank you MWJB for your suggestions !!! I think you just right about sqeel. it seems to be something about grounding or shielding .I checked and replace all componenets and i didn't find any problem.I don't have any photo (Maybe to few days-I'll borrow a camera).I'll check simillar threads about grounding or shielding (I'm newbie).

Regards !!!

[kiwisamourai]

hi,

i have the same problem with my champ and i can't find the solution!!!

ps: sorry for my bad english ( i'm french )