5F6-A new build issues -Resolved

[Stevewdewitt]

Hello all. I have searched the forum for this issue with minimal luck. I have just completed a 5f6-A from the Hoffman schematic/layout (http://el34world.com/Hoffman/files/Hoffman_5F6A.pdf). The issue is the volume is really low. It sounds the same in both channels. It doesn't sound bad either just really really low.

One of the new 5881's I bought had a dead short. I use a current limiter. I didn't bother to test it before I used it! My bad 

The current limiter caught it though. It was really bright.

I have measured the voltages with known good 6L6GC and 5881's

V1- 12AX7 P1:162VDC, P3:1.15VDC, P6:191VDC, P8:1.58VDC
V2 - 12AX7 P1:159VDC, P3:1.16VDC, P6:297VDC, P8:161VDC
V3 - 12AX7 P1:230VDC, P3:43.1VDC, P6:213VDC, P8:43.1VDC
V4 - 6L6GC P3:384VDC, P4:384VDC, P5:-42VDC, P8:19mV VDC
V5 - 6L6GC P3:384VDC, P4:384VDC, P5:-42VDC, P8:21mV VDC
V6 - 5AR4 P2:385VDC, P4:310AC, P6:310AC, P8:385VDC

With 5881s
V4 - 5881 P3:392VDC, P4:392.4VDC, P5:-43.2VDC, P8:18.6mV VDC
V5 - 5881 P3:392.4VDC, P4:393.7VDC, P5:-43.3VDC, P8:17.6mV VDC

I am using Magnetic Components trannies: PT 40-18073, OT  40-18000, Choke 40-18003

There was no smoke or anything out of the ordinary except the short in the bad tube caused the current limiter to light up super bright.

Any help, direction, suggestions are appreciated

EDIT: I did not use the pair of 100Ohm resistors attached to the filaments and ground. There is a CT for the 6.3V winding on the PT

TIA,
Steve DeWitt

[Willabe]

Welcome Steve! 

The power tube plate vdc's look 40 to 50vdc low. 
 
The power tubes are biased pretty cool, try taking them up to 35mA to 40mA and see if that helps. But when you bias them up higher the plate voltage will drop even lower.

Watch them as you do so you can see if they start to 'red plate'. (If they draw too much current they will over heat and the plat's will turn red. A small spot at 1st then larger as they get hotter.) Turn the amp off if they do and bias them back down, cooler.

They probably could take as much as 45mA to 50mA with out red plating.

There's a little math to bias them at 70% at idle but I'd have to go through my notes, 1 of the guys will probably post it. 

Edit; Changed low dcv #'s. Brad
 

[Willabe]

The rectifier tube, V6 vdc looks low are you sure you have a 5AR4 in there?

If 1 side is out on that tube then the vdc at pin 8 will be low. Have another to check with?

[Stevewdewitt]

I am sure I have a 5AR4 in V6

[Willabe]

Ok, Have another to check with? 1 side might be bad.

Or somethings not wired up right. (Or maybe a bad filter cap, but the limiter should have caught that.)

Could you also post the dcv's at each B+ filter cap node?

[Willabe]

If the 5AR4 is good, then it could be that the rectifier socket is a bad or not making contact on 1 of the tube pins  or it's miswired. 

That would cause 1 of the rectifiers in the tube not to be giving it's output to the B+ making it low. (5AR4 is a dual rectifier tube. That's why you see 2 plate structures in it.)

[eleventeen]

V1 12AY7: Here are 2 identical halves of a dual triode, fed from the same power supply node, thru "identical" 100K resistors, and sharing the same cathode bypass structure. They really should have the same amount of volts on them, or very close. It's not wrong for both to be 160ish or 190ish and it would not make the amp not work...but...something seems odd about that.


I agree, your B+ seems a tad low, though not so low that it would really dent the amp's performance. Got a 5U4 you can throw in there? "Same" basing diagram (not really, but OK for a test) the 5U4 nominally should drop about 30-35 volts more than a GZ34/5AR4. You are looking for about 430 VDC. Thus, if a 5U4 yields 390 (like you have now w/the GZ34) then your GZ34 is suspect.


A 5U4 pulls 3 amps filament current, while a GZ34 pulls 2. The tranny is probably rated for 3, but just to be safe, don't leave the 5U4 in there powered up all day long unless & until you confirm the PT can take it. For a brief test it is fine.

[sluckey]

V1 12AY7: Here are 2 identical halves of a dual triode, fed from the same power supply node, thru "identical" 100K resistors, and sharing the same cathode bypass structure. They really should have the same amount of volts on them, or very close. It's not wrong for both to be 160ish or 190ish and it would not make the amp not work...but...something seems odd about that.

They don't have the same cathode resistor. One half uses a 820 and the other half uses a 1500. That's why the voltages are different.

[sluckey]

The issue is the volume is really low. It sounds the same in both channels. It doesn't sound bad either just really really low.

A problem that I've seen several times on a new build of this type amp with these symptoms is a wrong value resistor on the cathode of V3. R20 will actually be a 470K rather than the correct 470 value. So check that. Also measure resistance (amp turned off) from V3 pin 8 to ground. Should be about 14K if all resistors in the cathode circuit are correct values. What do you measure? While you are poking at V3 insure that pins 3 and 8 are connected together.

I don't think your slightly lower B+ voltages are a factor.

[Stevewdewitt]

I have the same issue with a 5U4GB. I measured V3 pin 8 to ground and got 16.4K. R20 is 490Ohm. I am using carbon comp resistors except for the power resistors. Those are metal film. The bias is low but I can't get it any higher. Seems the bias pot doesn't have the range. It is 50K Linear.
 I will try a GZ34. I am going to switch out the preamp tubes as well. I got to hose from the same vendor as the power tubes.

I can measure all the B+ points tonight.

Thanks all!

[eleventeen]

Bassman 5F6 and 5F6-A show a shared 820 ohm and 250 uf on the schematics, maybe Doug's version as realized in his board splits them up.

[Willabe]

He does split them, 1 has a 820/220uF, the other a 1K5/22uf.

Shcemo link posted in his original post. 

[Stevewdewitt]

Here are the B+ voltages
A - 395VDC
B - 393VDC
C - 356VDC
D - 305VDC

I switched out all the tubes with known good tubes. Nothing. I used a JJ GZ34S. I re-measured every resistor. Seems all good. There is an issue with the bias circuit I think. I can only get the voltage up to 19mV.

TIA
Steve

[plexi50]

Can you supply a picture of your preamp section,board ,pots,including bias area?

[Stevewdewitt]

Here are some pics.

[Stevewdewitt]

more

[plexi50]

I can only get the voltage up to 19mV

What part of the bias are you probing to get 19mv? Pin #5?
 

[EL34]

You may need to change the bias range resistor to bias the amp properly


Check out this page in the library on biasing
http://el34world.com/charts/fenderservice5.htm

[Stevewdewitt]

I can only get the voltage up to 19mV

What part of the bias are you probing to get 19mv? Pin #5?
 

There are bias taps between pin 8 and ground.

[sluckey]

There is an issue with the bias circuit I think. I can only get the voltage up to 19mV.

This 47K resistor must be connected to the bias pot for proper operation. That is, unless you have a 47K mounted directly on your bias pot with one end connected to ground???

[Stevewdewitt]

This 47K resistor must be connected to the bias pot for proper operation. That is, unless you have a 47K mounted directly on your bias pot with one end connected to ground???

The bias pot is on the chassis under the board. I was tired of pulling chassises to rebias.  Hard to take a picture of. The wiper/center is connected to the 15k and cap.  The 47k is connected to one of the other ends of the 50k pot.

[sluckey]

Monitor the voltage on pin 5 of an output tube. Crank the bias pot to each end. What is the range of voltage on pin 5?

The bias issue is probably not related to your low volume issue.

[eleventeen]

What actual voltage are you delivering to the pin 5's of the output tubes? Or, what voltage are you measuring at the junction of the 2 qty 220K ohm resistors connected to those pin 5's that is the "distribution point" for your bias? If that voltage is [down] in the neg 60's, you are shutting off those output tubes.

[sluckey]

He's got 19mA flowing so they are not shut off.

[Stevewdewitt]

Monitor the voltage on pin 5 of an output tube. Crank the bias pot to each end. What is the range of voltage on pin 5?

The bias issue is probably not related to your low volume issue.

The range is -43-6 to -53.8. I don't think the low volume is related to the bias either.

[EL34]

There are bias taps between pin 8 and ground.

So you cannot get any higher than 19ma per tube, correct?
If that is correct, then you have too much negative voltage in your bias circuit
Changing the bias range resistor to a higher value will give you less negative voltage max and let the bias current go up
I provided a link for you in my post up above


The big blue 1k resistor is your bias range resistor

[PRR]

> range is -43-6 to -53.8.

With ~~390V on G2, you expect to want -23V to -39V of G1 bias to set a "reasonable" 6L6/5881 current.

I'd think 19mA would "play", and LOUD if you lean into it, just a touch rough when playing very soft. So I agree this is not your big problem.

[Stevewdewitt]

I changed the bias range resistor to 10k and now the range on pin 5 is -26.7 to -37.6 and I can bias properly. Still really quite.

Thanks all

[plexi50]

Go back over the cathode grounding and all resistor values for confirmation.  Then retrace all of your wiring connection points.
We all have made a simple mistake here or there and the worst part it is right in front of us.

[Willabe]

Link for a great way to double check your wiring;

http://el34world.com/Forum/index.php?topic=17701.0

[Stevewdewitt]

I will go over the layout/schematic/board and recheck evrything. Thanks for all the help. I will keep you updated.

[HotBluePlates]

Is that a master volume right next to the input jacks? It was hard to see in the picture how that was connected to the rest of the circuit, and to ground.

[Stevewdewitt]

Is that a master volume right next to the input jacks? It was hard to see in the picture how that was connected to the rest of the circuit, and to ground.

It is not connected yet. I just have it "stubbed" in. Good eye

[HotBluePlates]

That will be a bad spot for it; it should be all the way at the opposite end of the line of controls. You really don't want to bring late stage high level signals back over to input stage low level circuit points.

The pics didn't reveal obvious wiring issues or wrong parts values. I usually to a "tap test": When measuring plate or grid voltages with the amp on, there's usually a little spark and a thump in the speaker as the meter probe makes contact. Start from the output tube grids, and measure voltage. Then go to phase inverter plates, then phase inverter grids measuring voltages. As you continue moving backwards, each step towards the input jack from plate to grid should result in a bigger pop (due to tube amplification). Steps from grid to plate should have the same-volume pop, except when there's a volume control in between reducing volume.

You can often pin down which stage has weak gain in this manner.

[plexi50]

Let us know if you have confirmed all wire connection points. We all want to hear success. I myself at times expect the worst problem. But 90% of the time it turns out to be user error. That's one thing i learned on the forum and it is key to slowly going back over every wire connection. You will find the problem.  And when you do you will be mad at yourself for missing whatever it is. Don't hesitate to ask further questions. Their are no stupid questions. That's what makes this Forum so great. We are all family to a large degree.

[Stevewdewitt]

I have gone over every connection, more than twice, and I still cannot find the issue. I re-verified all component values. I am almost at the point where I want to yank the board and build another one but at this point I am suspicious of everything except the tubes. I verified every tube socket and pin as well. I tried a SS Rectifier. The voltages came up but still not really high. I also did as HBP suggested

I usually to a "tap test": When measuring plate or grid voltages with the amp on, there's usually a little spark and a thump in the speaker as the meter probe makes contact. Start from the output tube grids, and measure voltage. Then go to phase inverter plates, then phase inverter grids measuring voltages. As you continue moving backwards, each step towards the input jack from plate to grid should result in a bigger pop (due to tube amplification). Steps from grid to plate should have the same-volume pop, except when there's a volume control in between reducing volume.

It is really hard to tell by listening since the amp is so quite. I thought maybe V2 was the "weak" stage but hard to tell.

Any chance it is a bad OT? Choke? I ordered 2 of everything since I amp building two amps.

Thanks again all!

[Willabe]

If you have a 2nd set of iron for that amp, then since the B+ is coming in low, you could try and swap PT's and see what happens.

Just leave the wires long when you swap in the 2nd PT.

[HotBluePlates]

Could it be a bad OT? Normally I'd say no, because I've never seen a bad OT, much less a new one be bad. But it *has* happened to someone here on the forum.

Try swapping, but every time I had a new build not work properly it was because I mis-wired something.

[Stevewdewitt]

Try swapping, but every time I had a new build not work properly it was because I mis-wired something.

Me too. Every time. I will go over it again tomorrow.

thanks,
Steve

[Stevewdewitt]

Still no luck. I have replaced all the resistors/caps on V2 and V3 (they didn't need it but it seemed like a good way to look at all the points). I did it one thing at a time and then tested with no improvement.

It is about as loud as a 1 watt amp at the moment. I am going to have my son help me look over the whole thing. I need another set of eyes at this point.

Thanks,
Steve

[plexi50]

Use your DVM meter on continuity setting.  Check every wire but not just visually. Are you using your meter to each corresponding connection point? Pots and all tube sockets to the board. Recheck your power supply resistors are the right value/

[Stevewdewitt]

Use your DVM meter on continuity setting.  Check every wire but not just visually. Are you using your meter to each corresponding connection point? Pots and all tube sockets to the board. Recheck your power supply resistors are the right value/

I have been using the DMM. Checking continuity. All the pots. Values of all components. I am gonna get a new set of eyes on it. I can't see it.
Thanks

[HotBluePlates]

What kind of speaker jacks are you using? From your pictures, it appears at least one is a switched type (like a Switchcraft 12A jack; same as the input jacks).

Are they both switched? Or just one? (Hint: Only one should be switched).

Assuming only one is switched, plug your speaker into the one with the switch. How does it sound now?

[Stevewdewitt]

What kind of speaker jacks are you using? From your pictures, it appears at least one is a switched type (like a Switchcraft 12A jack; same as the input jacks).

Are they both switched? Or just one? (Hint: Only one should be switched).

Assuming only one is switched, plug your speaker into the one with the switch. How does it sound now?

only one should be switched? I am not an expert but I have used two switched jacks for outputs before and had no issues. I can remove one no problem.

[sluckey]

only one should be switched? I am not an expert but I have used two switched jacks for outputs before and had no issues. I can remove one no problem.

You can use two switched jacks but if the jacks are wired parallel you can only wire the switch lug of one jack to the ground lug and that jack should be labeled MAIN or SPKR and the other jack that does not have the switch lug connected to anything should be labeled EXT. You must always use the main jack. The EXT jack can only be used IF there is something plugged into the main jack.

If you connect the switch lug to ground lug on BOTH jacks, the only way to use the jacks would be to always plug something into both jacks. Otherwise, the empty jack will continue to ground the speaker signal from the OT.

A quick check to see if this is your problem is to simply insert a plug into the unused jack. The fix is to remove the jumper on the EXT jack.

Now if you have some other speaker wiring scheme, none of the above apply. Sometimes people will have an OT with multiple impedance outputs and they may use a separate jack for each impedance, ie, 4Ω jack, 8Ω jack, 16Ω jack. In this case you cannot use the switch lug on ANY jack. You only use one jack at a time.

Do either of these scenarios fit your amp? If not, tell us how your jacks are wired. Maybe post a pic of your jacks as they are wired now.

[Stevewdewitt]

Well I must start off by saying thanks to all! I am by no means an expert after 10 builds. It was the second jack. The second jack was switched.    HBP, good eyes! If you are ever in my area I owe you a beer. Again thanks all. It was as everyone said a simple mistake. I couldn't see it even though it was staring me in the face.

I will post pics when done.

Ever grateful,
Steve

[Willabe]

Great news, you guys found it! 

If you get a chance, I'm curious if the B+ changed/went up on the output tubes?   

[Stevewdewitt]

If you get a chance, I'm curious if the B+ changed/went up on the output tubes?

The B+ did not go up. Any suggestions there?

[Willabe]

No suggestions, I was just wondering. I don't think it's a big deal.   

[plexi50]

I make that same mistake every few years. Glad you have it working! Your voltages are fine.
I just ordered the same 325-0-325 power transformer.  I expect to see right at 400 B+ on my power tube plates.
Will let you know what i wind up with. Preamp and power tubes all draw a different amounts of current per manufacturer these days.