Fender PS 160 Vocal Amp Trany problems???

[billyboy]

This unit uses a trfmr and 6V6 for PI and it is bad so I'm parting it out. Pwr tranny is good, as is choke. OP tranny primary is at 760v on both 6550 plates no load. Can't test OP trany on the output/speaker side but I get a reading of .0-.4 ohm from gr to gr/yl and the same ohms from gr/yl to blk which is what I get probe to probe on my Fluke for ohms. What should I get for ohms on a good tranny.? The reverb tranny has 383 volts on the red to bl side and when I checked the ohms on blk to gr I get 1 ohm. Is that OK? Thanks

[6267]

The output transformer should NEVER be operated without a speaker attached. That 760v is your power tubes & transformer being very unhappy. You are lucky you did not see fire. With the amp off, and filter capacitors drained, you should not read an open circuit between any of the primary leads. The same goes for secondaries. There should be an open circuit between secondaries and primaries. You should consider reading basic tube amp theory before working with the circuits, and read up on tube amp safety. Those are very, very lethal voltages (DC +400 is instantly lethal in many cases, it takes much, much less AC, as little as 12v done "right"). Your transformer is likely fine unless you were getting no sound at all, and even then it's often not the transformer. Transformer problems are much rarer than tube, wiring, or user-error problems.

[billyboy]

Thanks 6267, I have rebuilt power supplies and done some trouble shooting along the way,in a dozen or so Fenders, half of which were my own, and other Epiphones, gibson, lectrolab, ampeg, rickenbaker, and a my stereo Fisher which someone had miswired. Repaired reverb tanks, rewired 12ax7 from 6eu7, 7868 to 7591. I have three books from Weber, 1 Torres, 1 Mitchell, hardly an electrical tube amp base of knowledge. I have hung around amp sites such as this reading and listening and belong to another. I also have another local TV man, 70 miles, who helps me and I send him repairs. I have a buddy who runs a shop, 200 miles, for 30 plus years who helps me and I send him work.And I will be the first to admit I'm a friggen idiot when it comes to most of this.

So I got the schematic and took it and the amp to my friend, retired 80 year old TV repairman who I play guitar with who has built a few of his own *ender amps and designed modules that went to the moon for Motorola, and I didn't want to screw the pooch on this on my own. This amp was given to me by a friend who said it didn't work 10 years ago and sat under the bench. We fired it up with no tubes in it and looked around for voltages. I have been told by more than one tech it's ok to do this with shorting jacks on speaker outs and no signal thru the amp, Please correct me if I'm wrong. As I said the PI tranny for the 6V6 was dead. All secondaries on pwr tranny seemed OK. Maybe a little high on heaters. So when we looked for resistance on the OP tranny we realized his meter leads were not good and he said to take it home and check again. He thought the resistance on the secondary might/could be as high as 200 ohm if good. Checking thru my Torres book he says to look for about .5 ohms on 4 0hm output,which is what this amp is, about .71 for 8 ohm output, and about 1 ohm for 16 ohm output. Well i'm getting .o -.4, ctr. to each side, varies, which is what I get when I go probe to probe on the meter leads. So I was hoping for some expertise, or thoughts, on this. I did check secondary to primary as you suggested so I think it's safe to assume this OP tranny is OK.Sorry if I wasn't more clear about all this in the first post. I tend to make word salad out of the correct nomenclature on occassion. Thanks, Much obliged.

[6267]

Sorry, I had thought you were measuring these voltages with tubes installed! It's perfectly ok to run an amp without the tubes, although in some cases it can stress the filter caps since the no load voltage is higher. I've never had a problem doing that though.

0.4 to 0 ohms on the secondary to center tap could just be indicative of very heavy guage wire on the secondary. Most meters start to get a little messy reading into inductances with very low resistance, I tend not to trust DCR readings on transformers with my cheap & somewhat fried meter. A reasonable output transformer test removing the transformer, connecting the primary to some AC source high enough to produce a readable voltage on the secondary (wall power if necessary, not really recommended) and reading the taps with the voltmeter.

[Gtr_tech]

This unit uses a trfmr and 6V6 for PI and it is bad so I'm parting it out. Pwr tranny is good, as is choke. OP tranny primary is at 760v on both 6550 plates no load. Can't test OP trany on the output/speaker side but I get a reading of .0-.4 ohm from gr to gr/yl and the same ohms from gr/yl to blk which is what I get probe to probe on my Fluke for ohms. What should I get for ohms on a good tranny.? The reverb tranny has 383 volts on the red to bl side and when I checked the ohms on blk to gr I get 1 ohm. Is that OK? Thanks

That PI transformer has dual secondaries....see the schematic. Are you sure you checked it right? That design is a bit more complex than your average Fender chassis. And ferchrissakes be careful in there...they're running those 6550s on the ragged edge at 700v on the plates. I'd only use *real* GE 6550A (yes the A version is different) tubes in there. You try'n use china's or most current production glass and It'll eat 'em right away.

Rich Koerner at Time Electronics is the guru of the PS series amps. If your PI xfmr is really bad, try him for a replacement.
http://www.timeelect.com/

[billyboy]

I'm not sure I would know how to do that,.. with variac, do you put hot on ctr tap of primary, neg to ground on chasis and then look for ac on the secondaries? Would I measure from grn/yl ctr. tap to grn and then to black with vom for AC. Say I put 10 volts on primary, what would i see on secondary? Thanks,

[billyboy]

Yes this is a bit more than your average fender. I'm pretty sure I checked it right. 330 volts into the red and nothing on the other end, blue, at the 6V6 plate. It also tests open with continuity check, red to blue. The other end of the transformer, secondaries, which supplies bias to the 6550 is at -45 v. as per schematic.

Maybe I'll send Rich a note and see if he's interested in these Xfrs as I have no intentions for using them. Thanks

[Gtr_tech]

Alrightty then....you have an open primary. Shame about the goofball xfmr...that coulda been a hell of a bass amp.

[jscballer]

i was wondering if the output transformers on these could be used without the driver transformer if a different PI was used, I picked up one of these and want to convert it to a more guitar oriented amp.

[Steve Conner]

Sure it could. You just won't get as much power with a regular PI: maybe 100 or 120 watts instead of 160. Probably just as well, it'll mean that modern power tubes have more of a chance of survival.

Did you already try it with the transformer PI and dislike the tone?

[DivAud]

Great device to check a output transformer is a Tenma 72-960 LCR meter...., pull the power and put the secondary on a load. Put the meter in the resistance mode and take a reading of the plate to plate taps of the secondary. A 50 watt amp should read about 2.4 K, a 100 watt about 1.8 K....or so ....

Shorted output Xfmr's will read real low 100 ohms or less....

This unit is a copy or/or made by the same people as a B&K unit one of my employees had...don'k know the B&K model number though.