67 Fender Showman low voltages Low Volume problem

[Gh987oo]

I wanted to ask for advice on an amp that I just recently received. All maintenance was completed on this amp but while playing it, I realized that the volume on this amp was about 1/2 (estimate) what it should be. I tried to bias the amp and it would not bias past 12 MA. I investigated voltages for this amp and I am finding that they are seemingly quite a bit lower than they should be. I took measurements of voltages with and without tubes. The voltages with tubes are the lower values. I just wanted to get some advice as to what I may be dealing with. I believe my power tubes are good and the amp sounds ok but just a much lower than expected volume from a showman.

Just as an aside when I did the maintainence (full cap job in the dog house and on bias and amp boards) I played the amp for a while and realized volume was lower than expected. Before I got it, the amp had had some maintenance done on it. New Screen and Grid resistors but still the old caps. What I didn't initially catch when I first changed out caps was that on the bias board, all the diodes had been taken out except for two (and the one diode going to the bias cap). The two were just laid across where there were supposed to be the 6 diodes, Each one of the two taking the place of 3 diodes. I subsequently changed that out to having the 6 N4007 diodes.

My measurements of voltages were as follows:

VAC going into bias board 311VAC  (with tubes 276V)
connecting to the 470 ohm resistor on Bias Board 50V
Heaters 2.8v (2.5V) 3.56 (3.1V)
Phase Inverter Voltages -51V (-43V)
411V (348V)
Power Tubes Pin 4 420V (358V) Pin 3 420V (358V)
V4 tube Pin 4 248V (215V) Pin 1 228V (200V)
V3 Pin 4 365V (302V) P1 411v (351V)
V2 P4 279V (243V) Pin1  278V   (245V)
V1 P4 277V (243V) P1  271 (241V)

These voltages appear to be quite a bit lower than on the specs for this amp according to the diagrams below. As an example the VAC into the bias board is supposed to be 345VAC yet I have 276 VAC !!!

The layout and Schematic are here:

https://cdn.shopify.com/s/files/1/2325/2729/files/Showman-AB763-layout.pdf?5926397210715640943

https://cdn.shopify.com/s/files/1/2325/2729/files/Showman-AB763-schematic.pdf?5926397210715640943

[EL34]

posted in wrong board... moving

[sluckey]

PT has to be high on the list of suspects. Measure the line voltage where the PT primary leads connect, probably at the fuse holder and power switch. Then measure the filament voltage across pins 2 and 7 of any 6L6. What are the two readings?

[Gh987oo]

Thanks for your reply. It is 102 VAC at the primary and 5.3VAC across pins 2 and 7

[sluckey]

Both readings are low. Now check the voltage coming out of your wall outlet. Measure between long blade and short blade.

[Gh987oo]

Coming out of my outlet it was 122 and I believe coming into the amp it was 121.

[Esquirefreak]

Are you using any type of current limiter between the wall and the amp? If so, that could be a culprit.

/Max

[sluckey]

So you have 121V coming in but only 102V going to the PT? Not much between those two points. Recheck those two voltages. DO NOT CONNECT ANY METER PROBE TO CHASSIS!!! To measure input voltage connect both probes to the courtesy outlet. To measure PT primary voltage connect one probe to the fuse holder and the other probe to the AC power switch.

[Gh987oo]

The VAC at the courtesy outlet is 122.5 and at the power switch when measuring from the switch to the tip of the fuse holder was also 122.5. I am including some photos as well. As you may be able to see in the attachments, the ground switch has been bypassed.

[Gh987oo]

I wanted to ask the best method for checking the power transformer. From what appears that there is 120 VAC going to the primaries for this power transformer. (originally I think I mis measured - thank you Sluckey for info in measuring the primary at the power switch. The 102 VAC reading came about because one probe was on the chasis when I got that 102 volt measurement.) Again under power it appears as if the high voltage wires that go into the bias board are seemingly way under the voltage that fender indicates (345 VAC) in the schematic should be there. (I am getting 276 VAC). However should I be checking these voltages with the power transformer disconnected or does it matter. Also should I be checking for resistances but again in order to do this should the power transformer be disconnected. Any help would be greatly appreciated. Thank you.

[Dave]

Yes, disconnect the secondaries, but you can still measure the secondaries with the primaries hooked up and the power switch on. You may have bad rectifier diodes (probably not).
What is the B+ voltage at the rectifier? Red probe to the point where the rectifiers come together and black probe to chassis DC voltage?


Dave

[sluckey]

Disconnect the two red wires from the bias board. Now measure the voltage between the two red wires. Should be about 700VAC. What have you? Be careful!

[Gh987oo]

Thank you for the comments and posts. Sorry I wasn't able to get back to you sooner but I am not always where my amps are. I was able to measure the VAC from the 2 red secondary wires(?) that go into the bias board and go through the diodes. I detached the transformer red wires and measured 635 VAC. From the layout schematic, I believe this should be closer to 700 VAC so that appears to be a fairly big difference. Also would this mean that my power transformer is underfunctioning or on its way out?? Again, I appreciate any comments or advice on this issue.

[sluckey]

635 is probably OK. Now connect one probe to chassis and measure the voltage to one of the red wires. Repeat for the other red wire. The two voltage readings should be about equal. What have you?

[Gh987oo]

Connecting to ground and to each of the red wires it is 318 on both red wires. Both of these secondary(?) wires read approximately 318 VAC.

[tubeswell]

Any model code on the PT?

[Gh987oo]

It is a Showman 125P34A which I believe was used in the Twin Reverb as well. It is I believe a 66 or 67.

[Gh987oo]

So if my voltages going into the bias board from the PT (red wires) are 318 VAC (although the layout and schematic indicate it should be 345 VAC) is that OK?? But when the tubes are in it measures something like 278 VAC which seems way off. Could it just be the tubes??? Or a problem in the biasing pot?? (As I mentioned I can't get these tubes hot the way it is set up. It goes to 12 or 13 milliamps when the bias pot is turned all the way up!!!) Any help would be greatly appreciated.

[mresistor]

Take the power tubes out. monitor the grid pin 5 and adjust the bias pot from one end to the other. What are the high and low negative voltages?


Because what you are saying is that there is too much negative bias voltage. The more negative it gets the less the tubes will conduct. The less negative the bias v gets the more the tubes will conduct.


It also seems like the tubes are conducting too hard which causes the plate voltage to drop. Or the PS load on the PT is dragging it down and the Plate Voltage is lower. 

[sluckey]

So if my voltages going into the bias board from the PT (red wires) are 318 VAC (although the layout and schematic indicate it should be 345 VAC) is that OK?? But when the tubes are in it measures something like 278 VAC which seems way off. Could it just be the tubes??? Or a problem in the biasing pot?? (As I mentioned I can't get these tubes hot the way it is set up. It goes to 12 or 13 milliamps when the bias pot is turned all the way up!!!) Any help would be greatly appreciated.

I think the PT is damaged and can no longer supply the necessary current. As a result, the PT voltage drops.

Measure the filament voltage with the amp in standby  and then switch to operate. What is the filament voltage in each case?

Measure the AC bias voltage on the PT red/blue wire at the bias board with the amp in standby  and then switch to operate. What is that voltage in each case?

[mresistor]

Some years back I had a Showman that exhibited similar problems and I had to replace the Power Transformer.
I'm pretty sure I used a ClassicTone PT but this is available: Hammond 290FX  https://www.hawkusa.com/manufacturers/hammond-mfg/enclosures/290fx

[Gh987oo]

Take the power tubes out. monitor the grid pin 5 and adjust the bias pot from one end to the other. What are the high and low negative voltages?


Because what you are saying is that there is too much negative bias voltage. The more negative it gets the less the tubes will conduct. The less negative the bias v gets the more the tubes will conduct.

It also seems like the tubes are conducting too hard which causes the plate voltage to drop. Or the PS load on the PT is dragging it down and the Plate Voltage is lower.

When doing this if I did this correctly (probe common on ground and red probe on pin 5) it was 45.7 DC on Standby and when operating it was 45 DC.

[Gh987oo]

So I did some of the additional testing as suggested. Measuring the AC at the bias on the red blue wire going into the 470 ohm resistor it was 47 VAC on Standby and 43 VAC when turned on. The filament voltages if I measured correctly (common probe on ground and using the red probe on the filaments for the power tubes pin 2 and then pin 7.) it was 3.25 VAC on pin 2 and 2.5 VAC pin 7 on Standby and when Operating it was 3 VAC (pin 2) and 2.3 VAC (pin 7).

[tubeswell]

So I did some of the additional testing as suggested. Measuring the AC at the bias on the red blue wire going into the 470 ohm resistor it was 47 VAC on Standby and 43 VAC when turned on. The filament voltages if I measured correctly (common probe on ground and using the red probe on the filaments for the power tubes pin 2 and then pin 7.) it was 3.25 VAC on pin 2 and 2.5 VAC pin 7 on Standby and when Operating it was 3 VAC (pin 2) and 2.3 VAC (pin 7).

The total of 5.3 VAC on the heater winding under load is a problem for reliable tube operation, and if the wall voltage is normal, it indicates the PT is bad. And the fact the heater voltage is dropping from 5.75VAC in standby to 5.3VAC under load indicates a PT load regulation problem (which alongside the low heater voltage, confirms the PT is bad IMO).

[Williamblake]

Does the powertransformer get warm? Maybe there is a large load that should not be there running in parallel with the amplifier?

[Gh987oo]

It does get warm but not overly so. Same as the tubes as well, they get hot but not very hot.

[mresistor]

If you want to be 100% convinced it's bad IMO  what is needed is a donor power transformer to substitute into the circuit in place of the original to see if the amp then behaves as normal. In my case I have a twin reverb power transformer sitting here and used it to test to confirm that the Showman PT was indeed damaged.  I don't do an install I just set the donor up on a 1x4 above the chassis and disconnect the original in the amplifier and then connect the donor's wires up for the test.


But Sluckey, Tubeswell and myself have said we think the PT is damaged.


If you have a thermal probe for your meter you can tape up the thermocouple end with tape onto the side of the PT and let it run for 1/2 an hour to see what the temperature is ..    I did that and it wasn't really that hot...  can't remember the actual temerature it reached ..  I think it was very warm to the touch.. Also the Showman PT is in the cabinet with very little cooling and this could be a factor in why they fail.

[sluckey]

If the voltage readings you posted are correct the PT is most likely bad.

[Gh987oo]

 I do have a Twin Reverb from the same time period as this Showman but for me (being a novice at best) I don't think it is a good idea to switch it out so I ordered a PT from Weber. There might be better PT (W022756) but the price was right ($75) when you go directly through them. (They initially charged a bit for shipping but they adjusted the price and indicated their shipping estimates were off sometimes.) As I initially posted, someone had done work on this Showman and put just 2 Diodes in place of the six that are supposed to be there! Not sure if that might have blown the PT. Anyway thanks for the advice and I will let you know when I get that transformer how things worked out. Regards and thanks to all who posted.

[Gh987oo]

Are you using any type of current limiter between the wall and the amp? If so, that could be a culprit.

/Max

Ok so I believe I found out what the problem was to some extent. Now please go easy on me, as again I am a novice and it is possible that when testing, as was suggested, there were times I had a current limiter set up and times that it was not in the circuit. But when I replaced the PT with the new Weber PT, it seemed that I was having a similar problem with lower voltages (with the current limiter plugged in) which was obviously prudent because I just put in a new PT. So it was then that I realized that (and I had a Duhhh moment) with the current limiter plugged in, it would draw current and without it in, the circuit currents would be near specs as indicated on the schematics. (Again, I am asking you to go easy on me). Well, voltages were near normal with the new PT (again without the limiter) so I also thought that the original PT probably wasn't bad after all. But I continued to have problems with the new PT and biasing. I was only able to get it up to 14 Milliamps with the bias pot turned all the way up. A little better than before but obviously problematic. I checked that bias balance pot to find that the 27k resistor on that pot drifted to about 33k. So I put in a 22k resistor and the bias pot was able to give me a high of about 40ma when biasing. So I biased a little down from there 34 to 35 ma with 6L6s. It gave me much more volume then when the tubes were cold at 14ma. Still a little lower volume than expected (and this is subjective) but perhaps not too much so. I replaced the new PT with the original and it behaved similarly with the new 22k bias resistor configuration. So while it did cost me a little money (fortunately that Weber PT was fairly inexpensive) and time the issues that I was originally having appear to be solved!! Again, thank you to all who had given me advice and support. If you have any questions or comments please chime in as I believe many of you have valuable information that people like myself are able to learn from.

[Williamblake]

Live and learn. Keep it up! Thanks for sharing.

[tubeswell]

Ohhh… kay.