Deluxe Reverb bias problem

[LoveChet]

I didnt want to throw in a new set of tubes just to see if the bias adjusted normal because I wanted to fix component issues if any first.

With the 6V6s pulled out the control grid measures -50V steady and the intersection of the two 220K measures -55v steady. Both measurements oscilate with some 60 cycle. With the tubes in the measurements are the same.

I thru in a set of used tubes and the bias on the first one I checked was way way down like 10ma so I am assuming the orignal tubes were the issue so I adjusted that first used tube back up to 25ma. The bias reading oscilated and I am assuming thats because the control grid bias voltage is oscillating.

Question 1: Are the -50 v and -55v control grid bias measurements correct ? I had read about it should be -35v.
Question2: Is the 60 cycle oscillation of the -50V bias voltage an issue ?
Question3: Assuming its a 6V6 issue is it ok to just throw in the new set of 6V6s and then adjust the bias current ?

Thanks

[LoveChet]

Sorry guys, post didnt give leading issue.

Deluxe reverb silverface was sounding real bad so I checked bias with weber bias probe. Measured 50ma and 40ma. No wonder it sounded bad. I adjusted bias down to 25ma and 20ma which was as low as it would go. Something wrong abviously. Then I started to measure control grid bias voltages which s where my thread started.

[pdf64]

-50V bias seems excessive but what’s the HT Vdc? Correct rectifier tube type?

I don’t understand how you can identify 60Hz oscillation on Vdc readings?

What is the amp’s service status, eg have its ecaps been replaced, ever / recently?

[Greg_L]

I guess first thing is to see why you can't get -35v at the grid leaks.

[LoveChet]

I'm not a performer so amp doesn't get huge amount of use. Power supply caps were all replaced probably 7 years ago. I have used a weber attenuator in the past but when it noticeably killed the 6V6s due to running hot I replaced them and amp went back to normal. 6V6s were new Tungsols a year ago and I recently replaced all 12AX7 and 12AT7 with new. I use only channel 2. I replaced the preamp tube with a 12AT7. At that point the amp sounded great.

Then the poor sound showed up and I believe the bias readings of 50ma and 40ma was an indication that something is wrong especially since the bias cant be adjusted lower than 25,20. Now I am trying to determine if the 6V6s just went bad or I have a component issue. I was reluctant to just throw in a new set of 6V6s since I didn't want to ruin them if components were bad. I have never had component issues in the past so components are all most likely stock except the power supply filters.

Sorry guys, I was an electronic tech years ago but never delved into amp repair so I don't know the lingo. What is 'HT' ?

Rectifier tubes is correct its a Sovtec 5U4G

So your saying the bias should be -35 ?

The oscillation I described as 60 cycle I am not sure what frequency it actually is. Its just that when measuring the bias with my Radio Shack analog voltmeter the needle fluctuates back and forth rather than remaining steady.

I havn't checked the 470 ohm screen resistors for value yet. I also havn't checked the 1500 ohm and the 220K resistors in the grid circuit for value. I also havnt checked the
.1 uf input signal caps by lifting.

At this point if you think -50 v bias is acceptable can I just put in the new tungsol 6v6s and see if I can adjust bias to 20ma, or do you think I have component issues and should locate that first ?

[LoveChet]

So Greg_L,

I guess you are saying that grid bias needs to be -35 and since its not, that's my issue, correct ? With the 6v6s pulled the readings are -50 so I am assuming that the 6V6s cant be the problem.

Thanks for the wiring diagram. Being new to this I don't know all the different versions of them but since my deluxe has .001K caps from grid to ground I was using a schematic that showed those. It is labeled as AB868.

[LoveChet]

Should I lift the .1 signal caps to see if that fixes the issue and gets me my -35 VDC?

470 ohm screen resistors read good
1500 ohm and 220K ohm resistors in grid circuit read good.
1oK resistor from bias pot to ground reads 5 k in one direction but charges up to 9k when reversing leads.

[Helmholtz]

The oscillation I described as 60 cycle I am not sure what frequency it actually is. Its just that when measuring the bias with my Radio Shack analog voltmeter the needle fluctuates back and forth rather than remaining steady.

It is possible to have 60Hz ripple on the bias voltage (half-wave rectifier). If so, you have a bad bias filter cap (25µ/50V). Mind "reversed" polarity when replacing.

[Greg_L]

So Greg_L,

I guess you are saying that grid bias needs to be -35 and since its not, that's my issue, correct ? With the 6v6s pulled the readings are -50 so I am assuming that the 6V6s cant be the problem.

Thanks for the wiring diagram. Being new to this I don't know all the different versions of them but since my deluxe has .001K caps from grid to ground I was using a schematic that showed those. It is labeled as AB868.

I'm no expert but I just built one of these amps. If it were me I would check everything that's involved to get -35vdc at the grids. The AB868 calls for -35vdc at the grids. I understand suspecting tubes and stuff, but getting -50vdc seems wrong and it's way off from what Fender specified, so look for the problem that leads to there IMO. The bias circuit is pretty simple. You've got a tap coming off the power transformer, a dropping resistor, a diode, a filter cap, a few more resistors, and a pot. Check all of those. The bias filter cap is most likely very old and could be leaky.

[Randall]

"It is possible to have 60Hz ripple on the bias voltage (half-wave rectifier). If so, you have a bad bias filter cap (25µ/50V). Mind "reversed" polarity when replacing. "

This ^^^^. It is important to remember -35v is just a rough target, not a hard truth. Somewhere on the schematic it reads all voltages are +/-20%. Add to that the fact that wall voltages are higher today then when the circuit was designed, so that makes everything 5-10% higher today. Add to that the fact that different sets of tubes will draw different current, and thus will need different bias voltages. So then you see that -35v is just a ballpark figure.

But I'm with Hemhlotz, you probably have a bad 25/50v bias cap. I like a 50/50v in that spot.

[Chuck H]

It is possible to have 60Hz ripple on the bias voltage (half-wave rectifier). If so, you have a bad bias filter cap (25µ/50V). Mind "reversed" polarity when replacing.

Second this^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you still have a problem with the bias current after that you should check the PI coupling caps for leakage.

[LoveChet]

Simple problem Solved, bad 6V6 and good lesson learned. When you adjust the bias pot its changes the control grid negative bias DCV, duh. That's how it works to set bias current.

When I had the tone issue, (loads of tinny highs, preamp volume acting crazy after 6), I used my weber bias probe for first time and found that the bias ma were 50ma and 40ma. Obviously I didn't know why they were so high so I adjusted them down as far as I could with the bias pot which was 25ma and 20ma. In doing so I didn't realize it, but what that did was, push the bias VDC way up to -50. Realizing what I had done, and hoping I just had a bad 6V6 as the original problem, I tried a different set of 6V6s in hopes that the bias readings now would be way low and they were. The -50 DCV and the different (good) tubes caused the bias ma to be way down to like 10ma. So I adjusted the bias back to 19ma which is where I want to start and lo and behold the control grid bias now reads -36.5. Evidently with a -35DVC bias the bad 6v6 was running way hotter than it should.

Thanks Pdf64 UK, Geg_L, Helmholtz Germany, Randall Florida, and ChucK H Pacific NW.

I will change the bias cap to a 50uf 50 VDC (with the proper polarity.)

In future I may change the PI caps also just for fun.

When will I ever get to play my Gretsch Country Gentleman 6122-1959. Problem is, if I don't like the tone of the amp I don't enjoy playing. Sound familiar ? I'm a tone freak, so was Chet. He would be ashamed of me bless his sole if he knew I waited until age 77 to open up my amp and fix things myself, adjusting bias as one of those chores . Chet most always fixed his own amps.

Thanks again

[Helmholtz]

bless his sole

I like this pun ^^^^

[pdf64]

I don’t think it feasible to visibly discern 60Hz oscillation on a voltmeter needle?
Rather the fluctuations seem more likely to be at a somewhat lower frequency.

HT means high tension, aka B+ in North America, principally high voltage delivered from the power supply to the amp; also subsequent distribution network taking / processing that current around the circuit.
So, in a DR, the stuff under doghouse, and the choke.

My thinking is that if the HT is greater than nominal, the bias must follow suit by a similar ratio in order to keep the idle plate current in check.
The use of a GZ34 rectifier would result in a higher HT than the stock 5U4.

The term ‘bias current’ is one of my bugbears; bias should refer to the (negative) voltage between g1 and cathode, which controls idle plate / cathode current. In fixed bias, the current flow in the g1 circuit is minimal.

[Chuck H]

The term ‘bias current’ is one of my bugbears; bias should refer to the (negative) voltage between g1 and cathode, which controls idle plate / cathode current. In fixed bias, the current flow in the g1 circuit is minimal.

I can get behind this. At least I'll try. I often use "bias current" instead of "idle current". But you're absolutely right.

There's no change without vigilance and Pete don't tolerate no ignance up in yore!