Rebuilt Amp- A couple problems

[SilverFox]

I rebuilt a Sunn Sceptre power amp section based on the Weber 6s100, see attached sch. The reason for doing this was to use the EF806 tube, which is more easily available than the 6AN8.

I've powered it up and found a couple problems. It does work so no smoke or major malfunctions but:

It was squealing when I first powered it up and swapping the grid wires at the AX7 tube solved that problem. But now the amp squeals when the NFB circuit is connected. by flipping the NFB sw. I've already reversed the grid wires on the phase inverter at R18,19 connections point and that fixed the first instance but now it appears related to NFB.

The presence circuit does not work; Wait, would it if there is no NFB?
I did change a value C11, from 1uf electrolytic, 100 VDC to .1 630 VDC since I didn't have a 1uf in that voltage rating. Other builds show a .022 on screen 2 so I figured it would be okay.

I also used Sluckeys version of the bias circuit, (attached), and then realized he had taken the voltage feed from the HV whereas mine is taken from a bias winding of about 52 VAC so perhaps the 18k resistor needs to be more like 28k. I can't seem to adjust the bias. I get a reading of 11 mv across the 1 ohm resistors- But the tubes are not red plating.

Another change is the use of a 12AX7 driver tube instead of an AU7 and 6L6GC output tubes. Don't think any of that will have an affect though.

I've twisted the output tube grid wires to keep things a little neater. I did this after reading that others had twisted them and had no problems with cross talk. ??


Any suggestions would be helpful.

The amp sounds okay but a little rough due to the lack of FB.

[ToneMerc]

Change the 18k bias tail to 33k for starters and then transpose the primaries on the OT to see if the squeal goes away, blue is normally the start.

TM

[sluckey]

The presence circuit does not work; Wait, would it if there is no NFB?

It would not work without feedback. I'm not sure how effective that presence control circuit is.

I also used Sluckeys version of the bias circuit, (attached), and then realized he had taken the voltage feed from the HV

No, my voltage feed comes from a 60VAC bias tap so the resistor values I used should work for you. Pull the output tubes and monitor the negative voltage on pin 5 of each output socket. Does that voltage change as you turn the bias pots? What range?

BTW, the bias pots on that Weber schematic are DANGEROUS since they will allow you to adjust the bias voltage to zero. Good choice to not use that circuit.

[SilverFox]

I'll check those things out later today and post some volt readings.

sluckey- I checked the bias with the tubes out prior to startup, did some basic checks.. The bias would only go to -40 but I figured that would be enough since I only needed about -35 for a plate voltage of 400 - 425. What surprises me is the fact there are no read plates at -11 bias.

Thanks a lot for the suggestions.

silverfox.

[sluckey]

11mV across a 1Ω cathode (pin resistor is very cold for a 6L6. That's not what I'm asking. I want to know what the negative bias voltage is on the grid (pin 5) and how much range you have when you turn the pots. You don't even need the tubes plugged in for this check.

You'll probably need the bias voltage to swing above and below -50VDC.

[SilverFox]

sluckey- I want to know what the negative bias voltage is on the grid (pin 5) and how much range you have when you turn the pots. You don't even need the tubes plugged in for this check

Just checked pin 5, tubes out, and was able to adjust the bias between -31.7 through -48.5 on both grids.

With the tubes in the bias is still adjustable from -30.5 through -48.5 on pin 5, both tubes.

I rechecked the B+ and that's at 419 with the screens at 416

I should perhaps clarify, the reading of -14 volts is taken from pin 8, which is connected to a 1 ohm resistor to chassis ground. I've been trying to adjust the bias by reading the -voltage across the 1 ohm resistor but only get a -14 volt reading.

I figure I'll get the bias adjustment right before changing anything else.

Thanks for the assistance,

silverfox.

[sluckey]

the bias is still adjustable from -30.5 through -48.5 on pin 5, both tubes.

I would increase the 18K to bring that range up to about -40 to -60 for 6L6s. 6L6s will be happy with about -50VDC on pin 5.

I've been trying to adjust the bias by reading the -voltage across the 1 ohm resistor but only get a -14 volt reading.

I'm sure you mean 14mV. That is very cold and contradictory to the adjustable bias voltage range you have on pin 5.

What is the voltage on pin 4 of each 6L6? Should be about the same as the voltage on pin 3. If pin 4 is good also, you may just have some weak 6L6s. Can you try another pair?

You mentioned that you rebuilt a Sceptre? But I see a laydown style PT. And did you use the original OT with U/L taps? Since you used the Weber schematic, what did you do with the U/L taps?

BTW, the only reason I used the resistor values shown in my dual bias circuit is because I had a pair of 10K locking pots and just built the bias circuit around those to provide the correct range of bias voltage for 6550s. If I had had a pair of 25K or 50K locking pots instead I would have built the bias circuit around those and the resistor values would have been different.

[SilverFox]

This amp build is a continuation of an ongoing project which was: Building a Sceptre style power amp section without the UL output and lower B+. I used the Weber 6s100 schematic attached, and made a few changes which are:

Changed C11, from 1uf electrolytic, 100 VDC to .1 630VDC since I didn't have a 1uf in that voltage rating. Other builds show a .022 on screen 2 so I figured it would be okay.

the use of a 12AX7 driver tube instead of an AU7 and 6L6GC output tubes. Don't think any of that will have an affect though.

I've twisted the output tube grid wires to keep things a little neater. I did this after reading that others had twisted them and had no problems with cross talk. ??

I'll wait to change the resistors since I was unaware you were referencing the original Sceptre schematic.

B+ is 420 and the screens are running about 416.

The output transformer is from a Univox 1221 and is not UL. Had 2 6L6GC output tubes running 660 VDC. I got to thinking I may have to change the bias feed resistors, R32, 33 to 100K. That might be why the bias reading is so low- 14nv,

Thanks,

silverfox.

[sluckey]

I was unaware you were referencing the original Sceptre schematic.

I've never been referencing the original Sceptre schematic. The only schematics I've looked at were the Weber schematic and the sniplet of my dual bias schematic you posted.

Have you tried another set of 6L6s?

The output transformer is from a Univox 1221 and is not UL. Had 2 6L6GC output tubes running 660 VDC.

That could be a factor for your low tube current. An OT designed for 6L6s working at 660V would likely have a much higher impedance than an OT for 6L6s operating at 420 VDC.

I may have to change the bias feed resistors, R32, 33 to 100K. That might be why the bias reading is so low- 14nv,

Those resistors will not affect the voltage readings on pin 5 nor the bias current readings on pin 8.

With 420V on the plates and 416V on the screen and a range of -30.5V to -48.5V on the grids, a good pair of 6L6s and a proper OT should operate from hot to red plating.

[SilverFox]

I've got some other 6L6's around to try.

I may also try the output transformer that came from the same amp as the PT. The OT was originally designed for 7868 tubes but when I used it in the past I didn't really like the sound of it. However, that was several years ago when I first started on this learning curve so I probably had other issues like using EL34s' with that OT. Recently starting to appreciate the relationship between OT impedance and plate voltage as in the past I thought the same impedance primary was used regardless of voltage. I've never been able to find the specs on this OT so short of putting a voltage on the secondary and calculating the impedance it's an unknown. It's likely an Ampeg compatible now that I think of it.

Thanks for the assistance. This should get me well into to the next diagnostic cycle.

silverfox.

[ToneMerc]

This amp build is a continuation of an ongoing project which was: Building a Sceptre style power amp section without the UL output and lower B+. I used the Weber 6s100 schematic attached, and made a few changes which are:

Changed C11, from 1uf electrolytic, 100 VDC to .1 630VDC since I didn't have a 1uf in that voltage rating. Other builds show a .022 on screen 2 so I figured it would be okay.

the use of a 12AX7 driver tube instead of an AU7 and 6L6GC output tubes. Don't think any of that will have an affect though.

I've twisted the output tube grid wires to keep things a little neater. I did this after reading that others had twisted them and had no problems with cross talk. ??

I'll wait to change the resistors since I was unaware you were referencing the original Sceptre schematic.

B+ is 420 and the screens are running about 416.

The output transformer is from a Univox 1221 and is not UL. Had 2 6L6GC output tubes running 660 VDC. I got to thinking I may have to change the bias feed resistors, R32, 33 to 100K. That might be why the bias reading is so low- 14nv,

Thanks,

silverfox.

I'm confused as I am not sure what you built. I assumed that you were using the KT88 per the Weber schematic and thus recommended the 33k over the 28k. However, the way things look you maybe need a 33k or you could change that single 1k to 15, 18 or 22k.

Good luck
TM

[SilverFox]

ToneMerc: The output tubes are 6L6GC.

silverfox.

[ToneMerc]

ToneMerc: The output tubes are 6L6GC.

silverfox.

I can't make out too well the Univox 1221 schematic I found but it looks like the dc high voltage scheme is 660 plates/330 screens in a doubler config like a MusicMan amp. The primary Z of a two tube MM amp in the 700/350 setup is 11k and I would expect this OT to be similar.

TM

[SilverFox]

I'm currently in the midst of troubleshooting the bias circuit. I've made some progress and been able to adjust it now, however it is very unstable and perhaps due to the faulty FB circuit, once I had it adjusted, it went out of control and one of the tubes red plated. Fortunately they were some Chinese extras I have around.

As soon as the FB circuit is engaged, ungrounded, I get a squeal. Could the impedance mismatch of the OT be causing the bias instability and FB sqeal? I've already swapped the grid leads on the PI, as it squealed when I first powered up the build. I just had a thought that perhaps the presence circuit is the problem so maybe I'll remove that and see if the squeal goes away.

I checked on the plate load resistance for the alternative transformer, which came out of a 7868 based amp and that primary should be 6600 ohms.

However, like I mentioned earlier the Univox OT does work and doesn't sound that bad. But, it would never be able to generate the full power of what I believe the PT should produce, 35 Watts. Previously when I used the Univox OT in a similar build it seemed to clip badly at near full output, (but that may also have been caused by the PI being overdriven in that build)... So I guess I'll strip it out after I get the bias and FB problems resolved.

Thanks for the suggestions ToneMerc,

silverfox.

[SilverFox]

It's working and sounds fantastic. With the FB engaged there is practically no background noise. With the FB disabled there is a little hum but the level is well below the output.

What was wrong-

When I powered it up the first time the squeal was present so I swapped the "grid wires" not the OT primaries. The squeal went away as long as I didn't try to use the FB. Now that makes sense, somewhat. I fixed it by swapping the OT primaries as was suggested in the beginning.

Tech Tip- When trying to set the bias be sure there is a load on the output transformer; I think, because until I connected a speaker I was unable to adjust the bias reliably. I believe the tube that Red Plated went bad too since after I changed that out I was able to adjust the bias to -32. There is also a flakey bias Pot that I'll change out tomorrow.

The presence control now operates.

I also found while trying to adjust the bias that it still floated around quite a bit but then realized it was due to the line voltage fluctuating. Can see that on the Variac meter.

Once I get burned in on this amp I would like to know more about how to overdrive the PI by changing some of the component values to get the swing voltages up. If that's the right concept.

Thanks ToneMerc and Sluckey for helping me get this amp running.

silverfox.