Help - Bassman 70 troubles - 60 cycle hum and hot V6

[zeppedled]

 Assistance is greatly appreciated! 

I have a 1982 Fender Bassman 70 I have been recapping and fixing up for someone. I have hit a dead end on tracing a 60 cycle hum as well as a V6 output tube that loves to go to town heating up (only when V5 has a tube in it).  The troble does not follow the tube, and does not matter if V1-4 are in or not. Oddly enough, other than these two issues, the amp powers up and actually sounds quite good.

Any thoughts on where to begin on this? V5 is perfectly fine, the amp produces sound (only when both power tubes are inserted) and it actually sounds quite good despite the hum. Also worth noting, it came in with a fried V6 socket, wiring, resistors...etc. Additionally, I replaced the output transformer for good measure.

I have attached some images, and more can be seen here: https://drive.google.com/drive/folders/1gqp1atgpYjdSuEgEa74Eyn1L2-FawHdV?usp=sharing


Here are the voltages from V5/V6

V5
Pin:
1 -43.2
2 HT
3 445
4 448
5 -43.5
6 448
7 HT
8 0



V6
Pin:
1 -43.2
2 HT
3 442
4 443
5 -43.5
6 440
7 HT
8 0

[Willabe]

I just worked on a Bassman 135, pretty much same amp, just double power, 4 x 6L6GC's.

These amps have a 4 and 8 ohm tap on the OT secondary, with special switching on the output jacks.

Looks like your new OT has 3 secondary taps, but you left the speaker jacks with their switching in place?

That switching has to go with a new OT. Or you have to use just 2 of the OT secondary taps, the 4ohm and the 8ohm would be the easiest.

     

[zeppedled]

The original OT had one output, I added a 3-way switch with the new OT to accommodate the 4, 8, 16 ohm outputs. Sadly this issue existed before the new OT.

[Willabe]

Edit; I had this wrong. It's similar to the 135 but in parallel not series. Different switch and wired different. It does switch OT secondary taps by plugging into the ext. speaker jack. See 2deaf's #12/#13/#15 replies.

The Fender schematic shows the same speaker jack set up and switching as the Bassman 135. I looked at your pics and I saw the same speaker jacks with the switching on them.

I understand that the issue existed before the new OT. I'm saying you can't leave that jack switching in place with the new OT with 3 secondary taps.

Edit; This is for the Bassman 135, but it's very similar to the Bassman 70, both have internal switching but with different impedance taps. But both amps want to see a 4 ohm load on the normal speaker jack and on the extension speaker jack.   

Here's how the speaker jacks work;

These amps came with a 2 x 15", 4 ohm JBL speaker cab.
Regular speaker jack, 4 ohm tap, with 1 cab, 2 x 8 ohm speakers wired in parallel = 4 ohm load.

Regular and ext. speaker jacks, 8 ohm tap, with 2 cabs, each cab; 2 x 8 ohm speakers wired in parallel = 4 ohm load, so 2 @ 4 ohm cab in series = 8 ohm load.

"1) The Bassman 135 is designed for speaker cabs of 4 ohm minimum
 impedance, for both the main cab and any extension cab.

 2) The Bassman 135 and the similar 135 watt Twin Reverb amps are
 unique among Fender amps, in that the jacks for the main and extension
 speakers are wired so that when both jacks are in use the cabs are
 connected in SERIES rather than parallel, and plugging in an extension
 cab also switches the speakers to a different tap on the output
 transformer (switched from the 4 ohm "center" tap to the "whole" 8 ohm
 secondary winding)."

And you can put a dummy plug in the regular speaker jack and run a single 8 ohm speaker cab in the ext. speaker jack.

"Another unique feature is that you can run either one or two 4-ohm cabs using the speaker and extension speaker jacks in the back, or just one 8-ohm cab from the extension speaker jack alone."

[zeppedled]

Ok, I get what you are saying now. Makes good sense. I am a bit baffled though why this thing had a hot V6 tube and a hum, I don't think this change will fix that. Perhaps I am wrong?

[VMS]

Are both tubes idling at 70% or how hot did you set the bias?

[DummyLoad]

pull all the preamp tubes - if still have hum, then look for issues with bias supply. if no hum with all preamp tubes out, then reinstall the preamp tubes one by one starting with the Phase Inverter (V4), then V3, then  V2 all the way back to V1. when you start to hear the hum after installing one of the tubes, that's where you need to look for issues - from that tube back to the input. 


--pete   

[Willabe]

This amp has a bias balance circuit, not a bias adjust circuit.

I'd go through the bias balance circuit. It looks like all the electrolytic caps are original. The bias caps could be bad/dying/dead, that could be your problem. And check the coupling caps that feed the power tubes for leaking dcv. If the coupling cap that feeds the tube socket that burnt up is leaking dcv, that would throw off the bias. Could be a bad solder joint that opens when the amp heats up.

And the hum could be coming from dying/dead electrolytic caps in the B+ power supply (PSU) or in the bias balance.

You have to find and fix why V6 keeps running away hot.

You changed the OT, so it could be a bias problem. 

[Willabe]

Ok, I get what you are saying now. Makes good sense. I am a bit baffled though why this thing had a hot V6 tube and a hum, I don't think this change will fix that. Perhaps I am wrong?

I never said that it would fix your V6 problem.

I said you can't leave the speaker jacks as they are wired now.

There 2 separate issues.

When you find and fix the problem with V6 the speaker jacks will still need to be changed/rewired.

Depending on how you set the impedance selector, what speaker load you use and what chassis speaker jacks you plug into, you could damage that new OT or burn it up.

Shouldn't I point that out to you since you didn't know about that?   

[zeppedled]

Shouldn't I point that out to you since you didn't know about that?

No I appreciate the information, I truly do. Looks like I need to keep messing with the Bias circuit to get rid of the other issue.

[Willabe]

These are the 2 bias balance lytic caps that probably need changing.

Doug sells a 100uf/100v Nichicon cap in his on line store.. (And all the other caps you'll need for this amp.)  Or you could also get an F&T 100uf/100v. Both of these would work. A little higher voltage and uF is better.

If it were my amp or I was working on it for a customer, I would recommend a full electrolytic cap job. Every lytic cap in the B+, bias balance and the preamp cathode (K) bypass caps.

I would also change the PI R's/C's to BF values.

And snip out the .01 NFB bypass cap that's in parallel with the NFB 820/1w series R. 

And if the amp is going to be used for guitar, not bass, I'd snip out the 220pF cap that's wired to 1 end of  the MV and going to ground. It's bleeding off high end chime/sparkle.

[Willabe]

And I'd probably take these 2 preamp tube 1st gain stage plate bypass caps out.

Tubnit has had good results using caps like these, but it has been in hi gain amps.

This amp is a bass amp and so they voiced to be deeper/darker in tone. This amp is not hi gain, those caps might bleed to much of the high end signal off to ground if used for guitar.   

[2deaf]

I just worked on a Bassman 135, pretty much same amp, just double power, 4 x 6L6GC's.
These amps have a 4 and 8 ohm tap on the OT secondary, with special switching on the output jacks.
Looks like your new OT has 3 secondary taps, but you left the speaker jacks with their switching in place?
That switching has to go with a new OT. Or you have to use just 2 of the OT secondary taps, the 4ohm and the 8ohm would be the easiest.

The 70W Fenders used an OT with 2 Ohm and 4 Ohm secondary taps.  With only the main speaker plugged in, it is connected to the 4 Ohm tap.  With speakers plugged into the main and the ext. jacks, the speakers are connected in parallel and to the 2 Ohm tap.  This is different than the 135W which hooks two speakers in series and to the 8 Ohm tap. 

[2deaf]

The original OT had one output, I added a 3-way switch with the new OT to accommodate the 4, 8, 16 ohm outputs.

The original OT had two outputs, 2 and 4 Ohm.  The 4 Ohm tap (Green) goes to the board where it connects to the 820r/0.01uF NFB combo.  A white wire goes from this junction to the leaf jack (Ext.).  A 2.2K resistor goes from the leaf jack to the line output jack.

You need to run a wire from the 4 Ohm tap on the impedance selector to the 820r/0.01uF NFB combo and a wire from this junction to a 2.2K resistor going to the line output jack.  The leaf jack can be disassembled and re-assembled so that the switching is disabled and then you can use one of the terminals for a tie point for the 2.2K resistor.  The hot on the leaf jack could then be connected to the hot on the main speaker jack.

[Willabe]

The 70W Fenders used an OT with 2 Ohm and 4 Ohm secondary taps.  With only the main speaker plugged in, it is connected to the 4 Ohm tap.  With speakers plugged into the main and the ext. jacks, the speakers are connected in parallel and to the 2 Ohm tap.  This is different than the 135W which hooks two speakers in series and to the 8 Ohm tap.

His amp may be different?

I thought the Bassman 70's came with 4 x 10", that could have been wired for 2 ohms. So what you say would make sense.     

But the back face plate says the same as the Bassman 135, 4 ohm min. on each speaker jack.

The pic below is from the pics he posted.

Edit: See 2deaf's post's above, #12/#13 and below, #15 and mine below.

[2deaf]

His amp is exactly like the schematic.  I can see he has a transfer circuit with his leaf jack while the 135's have an isolated switch on their's.  The Bassman 10 (70W), Bandmaster Reverb, Pro Reverb, and Super Reverb used the same transformer for their 70W versions.

[Willabe]

Thanks 2deaf for explaining this, I see it now. 

The 70W Fenders used an OT with 2 Ohm and 4 Ohm secondary taps.  With only the main speaker plugged in, it is connected to the 4 Ohm tap.  With speakers plugged into the main and the ext. jacks, the speakers are connected in parallel and to the 2 Ohm tap.  This is different than the 135W which hooks two speakers in series and to the 8 Ohm tap.

I'm sorry, I missed the speaker jacks on the Bassman 70 in parallel and the Bassman 135 in series. Different switch, but still switching between 2 OT secondary taps.

So, the speakers that came with the amp are wired for 4 ohms? And if you add an extension speaker cab, it should also be 4 ohms too. That gets you down to 2 ohms, 2 @ 4 ohms in parallel = 2 ohms.

He still needs to do something with the way the speakers jacks are wired, now that he installed an OT with 4/8/16 ohm taps and an impedance selector switch.

Or be very careful what they plug into which speaker jack(s) at which impedance is selected. Be very easy to get confused with so many options.     

[2deaf]

He still needs to do something with the way the speakers jacks are wired, now that he installed an OT with 4/8/16 ohm taps and an impedance selector switch.

I'm thinking connecting an Ext. jack in parallel with the Main jack and no switches on the Ext. jack.  Or remove the Ext. jack altogether and put a hole plug in.  Whoever uses it just has to know the total impedance needs to match the impedance selector.

[Willabe]

After you get V6 socket from over heating, there is a hum balance pot for the heaters. 

Adjusting that might help a lot. If you plug a guitar cord in without a guitar hooked up to the other end, it will act like an antenna and pick up noise, stray signals, from the air. Easier to adjust the hum pot this way. 

I still think the B+and bias caps need to be changed, they can cause hum.