More help needed on my Showman Reverb

[krankguitarist]

Unfortunately since I no longer have a camera I can't use pics in this thread, so I'll try to describe my problem as best I can.

I'm having a problem with my reverb doing this buzzing act, and as I'm poking around inside the amp, somthing in there is bothering me.

There's a bigass 150 ohm 7 watt resistor between pin 8 and ground on all four of my power tubes.

On top of that, there's this very strange-looking contraption soldered to the side of the chassis. Its got about four terminals on it, and three white mallory caps soldered between the terminals (one cap between terminal 1 and 2, second between terminal 2 and 3, etc). Each terminal is connected to a wire that goes to pin 8 on each of the power tubes.

None of this is on the schematic. All of this is making my head hurt. Any ideas as what the previous modder may have been trying to accomplish by this?

 

[sufferinrewind]

Re: More help needed on my Showman Reverb

No idea what the dude was trying to do, but I think you might need some asprin and a nap. It seems like that amp has given you nothing but trouble since you got it. :blackeye:

I hope it'll be worth it when you're finished working on it. It would suck for you to have gone through all that trouble and put all that time and money into it for you to end up unhappy when it's all said and done.

Good luck with it, man.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Eh, I know I may bitch and moan about it, but the headaches come more from wanting to understand what the dizzle is, rather than the act of having to fix it. I'm actually having a lot of fun fixing this thing.

I know, I'm a geek.

Just found another little discrepancy though...there's another white mallory cap on the circut board that doesn't match up with the schematic.

here's a pic:

That white mallory to the right is a 25 uF 25V cap, and the schematic says its supposed to be a 5 uF 25V cap.

Hmm...however...the blackface schematic seems to have a 25uF 25V cap there.

Some of the mods on here seem to have been done in an effort to "blackface" the amp a bit. But some of the others are just perplexing as hell.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Just one more thing...I dunno if this is gonna sound silly or not. Eh, its just a question.

I'm checking out the cardboard Mallory caps on there...and I decide to ohm em out. I have no idea what that'll prove, but I'm doin it anyways. These things are two 50uF 50V caps in one can, so they've got a single lead on the negative side, and two leads on the positive side.

So, I check out the first cap with my DVM. I get an even reading from the "negative" side to both leads coming out of the "positive" side. Then I go onto the next two (seen in the picture), and the leads on the right side of the "positive" end on both have significantly higher readings than the leads on the left side of the "positive" end.

Now...here's where things get interesting (in my mind at least). The cap on the left's "right" lead goes to pin #3 and #8 on V3 (which drives the reverb, which has been buzzing a lot recently).

The cap on the right's "right" lead goes to pin #3 on V6 (which drives the vibrato, which takes a *very* long time to warm up on this amp, usually about 2 minutes or so after the amp itself has warmed up).

I dunno...I'm just making theories. Feel free to shoot em down if you wish . Though, If there's somthing to what I'm saying, I'm thinkin it might be a good idea to replace em.

 

[kmcguitars]

Re: More help needed on my Showman Reverb

There's a bigass 150 ohm 7 watt resistor between pin 8 and ground on all four of my power tubes.

WTF ! Pin 8 is your cathode. Stock there is just a braided wire to GRN on pin 8. It's common to install a 1 ohm 3 watt resistor on pin 8 to check your bias. The only reason I can think of to have a resistor on pin 8 is for reading bias. I guess a 150 ohm could work for this pupose? But I Imagine you'll have to do some math to get a reading?

 

[ES350]

Re: More help needed on my Showman Reverb

If that resistor is for bias reading, it should be a 1 ohm...
That Mallory 25 mike is stock...my '68 Twin has that value. Lots of discrepencies between schems and actual layout in early SF stuff.
The cap array to all pin 8 may be snubbers...they went to 2000pF eventually.
Might want to replace those 25/25 caps since you're in there...
None of this has anything to do with noise in the reverb circuit...might want to use the BF era Twin reverb circuit as a template...

 

[krankguitarist]

Re: More help needed on my Showman Reverb

If that resistor is for bias reading, it should be a 1 ohm...

Thats what I was thinking. I doubt it's for mesuring bias.

The cap array to all pin 8 may be snubbers...they went to 2000pF eventually.

That would make a bit more sense...though I'm considering just pullin the whole thing out and wiring pin 8 to ground just for the sake of getting it stock again.

As for those 25 25's...I found a couple of resistors wired in parallel w/ em that would account for the different readings I was getting. So, nuts to that theory :smack: .

I'll take a look at the schematic...I might just have re-build the reverb circut to get it right. The buzzing is unholy.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Well, I yanked those resistors and the little "cap array". I wired pin 8 on each of the power tubes to ground, and voila; I can't tell any difference.

Eh, the reverb is still buzzing away though. I'll look through the schematics and see if I can find anything interesting.

 

[Scott_F]

Re: More help needed on my Showman Reverb

Sounds like a silver face circuit with those big resistors.... Check the jack for the reverb pedal if there is one. Check the pedal side as well. My first instict says that little RCA jack is prone to issues.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Checked out the RCA cables for the pedal, but no luck. Same with the reverb input/output RCA jacks and the cables.

The tubes are brand new, but I tried swappin those out anyways. Still buzzin.

 

[Scott_F]

Re: More help needed on my Showman Reverb

Well, buzzin' is quite often a grounding issue. So, what's changed on the amp recently?

 

[kmcguitars]

Re: More help needed on my Showman Reverb

If it was my amp, I'd change all the 25uF25v electrolytes.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Well, buzzin' is quite often a grounding issue. So, what's changed on the amp recently?

The change I made that seemed to bring out the buzzing was changing the bias circut from this:

To this:

http://www.effulgencemusic.com/sdpictures/bffender.gif

The way it was wired up before was pretty perplexing as well. It was hooked up just like a bias adjust pot, but it had a pair of 10K resistors coming from the left and right lugs that met up with a 100K resistor on a little terminal that the previous modder soldered onto the chassis. That 100K resistor met up with that first blue cap you see in the pic.

I took out those 100k and 10k resistors, and made it look a bit more like a BF bias circut. Before the power tubes were running cold as all hell, now they're biased right. But...I didn't notice any humming in the reverb until I did that little mod. Which is strange considering that it doesn't seem to have anything to do with the reverb circut? I can't figure it out.

If it was my amp, I'd change all the 25uF25v electrolytes.

I think thats the next step for me. I've been reading a few articles on hoffman amplifiers, and I'm thinkin about doin the "mods" on this page:

http://www.hoffmanamps.com/charts/fenderservice1.htm

As well as adding a 3-prong power cord. Its got a 2 prong cord in there right now.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

One more question. I'm gonna pick up some 100k and 47k resistors as replacements for the 100k and 47k "plate load" resistors in there. Figure I might as well while I got the thing open.

The guy at hoffman amps says he likes to replace 'em with 3 watt metal film resistors. I wanted to know what kind of resistors you guys would reccomend.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

I just got done installing a new power cable on this thing. It's grounded and all that shizzle. Reverb's still buzzin away. Not exactly like I expected that to solve my problem though .

Was a real pain in the ass getting that strain relief bushing back in though. Sheesh.

Also, from looking at that little cap array, the caps on the little board i pulled out were dated to 1968 as well. Which means that either it was done at the factory that way, or somebody modded it when it was pretty new (which I doubt).

Eh, either way, the caps are gone now. Didn't seem to make much of a difference.

Edit: Ah! one more thing. I ended up buyin a 2'X4' panel of birch plywood over at home depot, gonna make a new front-panel for this thing since the old one has those holes cut in it for the 8" speakers. I'll use the rest and the original front-panel to make a nice little 2X8 cab...use that for my keyboard. All I'll need is somthing to power it with...

 

[Jeff Seal]

Re: More help needed on my Showman Reverb

Just hold on fer a second there!!!..(in my best Texas accent!)

If you're getting a hum from the reverb ONLY when the reverb knob is turned up, not a problem... either the cable that goes from the output of the reverb tank is bad AND/OR the ground connection of the cable at either end is bad...

Try the following... unplug the wire labeled Reverb Output and measure the ohm reading between the center terminal of the RCA jack and the shield (or outer terminal). You should get a reading of about 150-250 ohms. Since I'm 95% sure you're not gonna get that reading, unbolt the reverb tank and check both ends of the cable for continuity. The center terminal on one end should read less than 2 ohms than the other end and the outer terminal should be the same from one end to the other... oh, and while you're there make sure the cable that comes from Reverb Output on the amp goes to Output labeled on the tank. If the cable is good, you should get the 150-250 ohm reading between the terminals in the tank itself... if not replace the tank.

Most of the time simply wire brushing or cleaning the terminals will resolve most reverb issues...

You do have my permission to "slap" whoever tried to convert your amp to "cathode biased"..... now you know what I get in all day at the shop!
Cathode biasing an amp that already has a negative grid feed just runs the tubes "colder"....... (sarcasm inserted for the following!)

...BUT it's cathode biased!!!!!.......THAT's where the TONE'S at!!!!:laugh2: :laugh2: :laugh2: ... you would not believe what I have seen done to amps!

Jeff Seal

fwiw.... let me know what you discover.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Try the following... unplug the wire labeled Reverb Output and measure the ohm reading between the center terminal of the RCA jack and the shield (or outer terminal). You should get a reading of about 150-250 ohms. Since I'm 95% sure you're not gonna get that reading, unbolt the reverb tank and check both ends of the cable for continuity. The center terminal on one end should read less than 2 ohms than the other end and the outer terminal should be the same from one end to the other... oh, and while you're there make sure the cable that comes from Reverb Output on the amp goes to Output labeled on the tank. If the cable is good, you should get the 150-250 ohm reading between the terminals in the tank itself... if not replace the tank.

Just checked the output jack on the reverb tank, it ohmed out at about 205 ohms. The input jack comes out at 1 ohm. Though...I'm a little confused about the rest. I tried measuring from the center terminal of the input RCA jack to the center terminal of the output RCA jack and I didn't get any reading. Same with the sleeve. Am I supposed to be getting a reading?

As far as the cable I'm using, It aint exactly the prettiest thing in the world, but it does work...switchin it out with other RCA cables doesnt seem to do anything to help the buzz.

You do have my permission to "slap" whoever tried to convert your amp to "cathode biased"..... now you know what I get in all day at the shop!
Cathode biasing an amp that already has a negative grid feed just runs the tubes "colder"....... (sarcasm inserted for the following!)

...BUT it's cathode biased!!!!!.......THAT's where the TONE'S at!!!!:laugh2: :laugh2: :laugh2: ... you would not believe what I have seen done to amps!

Jeff Seal

fwiw.... let me know what you discover.

There we go! I'm not exactly sure what that all means, but at least I've got a name for it now! Thank you for solving that little mystery .

 

[krankguitarist]

Re: More help needed on my Showman Reverb

One more n00bish question...If I unplug the reverb and switch it on, I still get some buzz (though it sounds a bit different). Is that normal?

 

[krankguitarist]

Re: More help needed on my Showman Reverb

An update: I've replaced 5 out of the 7 electrolytic caps as of right now, as well as the bias cap. I'm holding out on the last two till I get another 25uF 25V cap, since one of the 7 caps I ordered was faulty. :smack:

I've also replaced a few of the resistors on the power tube sockets...all according to that hoffman amps page.

On a cosmetic note, I ended up cutting that new "faceplate" to fit, spraypainted it black, covered it with the old material, slapped that fender logo on, and it looks friggin sweet now. No longer do I have those big holes in the front where the 8" speakers used to be.

The amp's still running real quiet, except...the reverb :yell:.

Another thing I noticed: When I turn off the reverb via the footswitch, and turn up the reverb knob, I end up getting a slight "hiss" in the signal. When I turn on the reverb though, it does get considerably worse.

 

[krankguitarist]

Re: More help needed on my Showman Reverb

Another little update, I ohmed out a few of the resistors in my reverb "recovery" circut, and a few of em seemed like they had drifted pretty far off. I'm gonna pick up some replacements for those, see if that helps things any.