Help with Chinese 5F6-A Kit

[sstudley]

Hello,
I am very new to this forum. I am hopeful I can find some help here with my 5F6-A kit, that I have recently finished assembling. First, let me come clean, if I knew then what I know now, I would have bought a Hoffman 5F6-A kit hands down. Instead I bought a Chinese 5F6A kit. I assumed buying a Chinese kit would save me lots of money I could use to buy better parts and upgrade the kit and so on. The kit came with no and I mean NO instructions. I am comfortable with working on electronics, so it didn't bother me that much. Well, although I have done a fair bit of electronics repair work over the years, it has primarily been solid state. Only recently have I got up the nerve to tackle tube projects. So this is my first scratch build vacuum tube guitar amplifier build. Since I had no BOM, schematic or layout, I grabbed the schematic from Rob Robinette's site and the layout from Weber's site. Hoffman's layout was considerably different from what came in my kit, the weber turret board pattern was much closer to the turret board in my kit. So I used those.

In the end I have a very loud 60 cycle hum. The Blue with red stripe bias tap from the PT is less than -2.00 VDC. All the B+ variants are running at least 20% or more low, even though the B+ without the Standby being thrown is 425 VDC, exactly what the schematic calls out. The amps is burning 33 watts when power is on but still in Standby, 220 watts when Standby is thrown!

Here are a few questions I have:
At -2.00 VDC is it safe to say the output tube bias is significantly high? Do I have the wrong wire? The hum would go away if the bias were fixed?

I have swapped out the supplied 5881 output tubes from the kit for some decent quality 6L6’s. I believe the original 5881 that came with the kit are likely destroyed.

Please keep in mind, in light of not having any proper instructions, layout or schematic with this kit, I am making educated guesses wrt wire taps with both Power Supply and Output Transformers. I am using the yelo wire for my 8 ohm tap off the OT, along with the black common. The Green/Yelo and Red/Yelo taps from the PT go to ground along with  the Green/Yelo wire coming from the mains.
 
Thanks in advance,
Steve

[sluckey]

You have a bias voltage problem...

Pull the output tubes and don't put them back in until you can measure approx -40 to -50 vdc on pin 5 of the output tube sockets.

The red/blue bias wire from the PT should measure about 50VAC. It ain't DC! Disconnect that wire and measure the AC voltage between the red/blue wire and chassis. What have you?

[sstudley]

With the negative dmm lead attached to my preamp ground bus, I read 54 VAC.

[sluckey]

OK. So the PT is good. That only leaves 5 components. And your wiring. Post some hi-rez pics of the inside of your amp. Maybe we can spot the problem.

This is the schematic I will be using for reference.

     https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a.pdf

Can you post a link to a website where you bought this amp?

[sstudley]

Thank you for your help! Here is the URL to Mable Audio, the Chinese Vender I purchased my kit from: http://www.mableaudio.com/en/productview.asp?sid=649.

Here are the pictures, to get things rolling. I expect additional zoomed in images might be requested.

[sstudley]

Thank you for your help! Here is the URL to http://www.mableaudio.com/en/productview.asp?sid=649.
Here are the pictures, to get things rolling. I expect additional zoomed in images might be requested, this time a little higher res.

[sluckey]

Please keep in mind, in light of not having any proper instructions, layout or schematic with this kit

That link clearly states that a schematic and wiring diagram are included with the kit. If you really didn't get one you should contact MableAudio and get one. I see they have a chat feature on site. You can probably get a link to download documents or have them emailed to you.

I need to see a HI-REZ pic that clearly shows the bias diode and components for the bias circuit. I can locate the bias diode in one of your small pics but can't tell anything else.

[sstudley]

A couple of bits of information that come mind after looking at the layout you provided a url for. I am not using a selenium rectifier. Not sure if that is relevant, just a note.

I am building a 2 x 12" combo. The speakers are Celestion G12-65 rated at 16 ohms.

NFB is governed by 57k resistor, based on Rob Robinette's schematic for 8 ohm speaker load.

I am not using the ground switch.

Thanks again for your time.
Steve

[sluckey]

A couple of bits of information that come mind after looking at the layout you provided a url for. I am not using a selenium rectifier.

Of course not. I said I see your bias diode.

[sstudley]

Sorry, I didn't mean to insult your intelligence. Being new to the forum, I was thinking more information, however trivial or obvious was a good thing. I will try to stick to relevant information  in the future.

I have attached a cropped and zoomed in image of the bias circuit. I hope this helps.

FWIW: I did attempt to contact Mable Audio about the missing documentation. They seemed to have little interest in helping me. Admittedly, I gave up rather quickly.

[sluckey]

It's very hard to insult me. Don't worry about it.

That's a very nice pic. There doesn't appear to be anything connected to the anode of the diode (top end in your pic)?

[sluckey]

There needs to be a short jumper between the top end of the diode and the top end of the 15K resistor

[sstudley]

I soldered in a jumper wire across the two turrets, diode to 15k resistor. I powered it up without the output tubes installed. I had my dmm set to VDC with the negative lead attached to the preamp ground bus and pin 5 (looking for -40vdc - -50vdc as you had mentioned in an earlier post). I shut it down after seeing the voltage climb past 400 vdc and just before doing so, a small amount of smoke became visible. Looks like the 1 watt 470 ohm resistor across pins 4 & 6, on the (nearest to rectifier tube) output socket. Likely they both got hot, just shut it down before the other socket pins got to the smoking point.

[sluckey]

Pull the rectifier tube and don't put it back in until the bias voltage is correct at pin 5 of the output tubes. Try it again.

It appears there may be multiple errors in this build. Good news is that it appears your board mostly follows the layout/schematic I linked to. Do you have any jumpers installed under the board? Anywhere, not just the bias area?

Do you have the red/yellow PT wire connected to chassis?

[sstudley]

oh my, did I screw up! Long story short. After putting the build together I ended up with what was hopefully, just the bias issue, you just fixed. However, during the build that first output tube ceramic part of the socket fractured. I decided to replace it. Thinking it was in some way related to my (bias) problem. In wiring everything back up to that socket, I reversed the pin order, for everything. I am so sorry for wasting some of your time. You certainly helped me find both issues and I am very grateful! I really do feel like a dumbass. My mind was so filled with blaming the kit, quality of parts, lack of documentation... etc. I may still have problems yet, we won't know until I rewire that socket. I'll post again, but it will be a few minutes.

[sluckey]

No waste. I enjoy this stuff. And you picked up some more experience. Win/win. Good luck with the amp.

[sstudley]

ok, want a good laugh? 
The pin order was fine... the resistor smoked because??? It was 4.7k instead of a 470 ohm as was the case in the other! All is well, my Bassman is working fine now. Thank you so much! My nerves and general fear of saying something or doing something stupid causes me to jump to errant conclusion(s)! The amp sounds pretty. Oh and the funny part you say? That's for me to know and you to guess, if you want!

[vampwizzard]

hey now! you got good experience! and started thinking through the problem.. which is the real win.