Kustom amp question.

[digi2t]

Yeah, yeah, I know... this is a stompbox forum. But since my electronics familia resides here for the most part, and I know that there are some fearsome amp gurus in here, here goes...

I picked up a Kustom III Lead SC head the other day (thanks Mark!). It's a 130 watt amp, looks to be originally out of a combo. Two channels, one with a drive knob to dirty things up if need be, and second channel is straight clean. VERY loud, and VERY clean. The drive section sounds pretty good as well, very good grunt through my 2x12 Texas Heats cab. Almost worth isolating for a pedal, like Jimi's Suzie Q project, but I digress.

Now... the problem.

This is a master volume unit, and next to the master volume, there's a knob labeled "Voicing". Both preamp sections have their own EQ sections (treble with treble boost pull switch, mid, bass), but the voicing section sits between the preamps, and the master volume. The voicing section is it's own board in the unit, being fed by both preamps. The voicing pot is a dual pot, one pot controls the... contour?, for lack of a better word, and the other feeds the amount of voicing required to the master volume.The master volume feeds the power amp section.

The problem is that there is a strange interaction between the voicing, master volume, and treble controls (of both channels). Dialing the voicing to 10, counter intuitively, is the least amount of output from the voicing section. At this point, I can crank the master volume to 10, and set the treble to any position I want, and the thing sounds great. Now, roll back the voicing pot (inject more signal from the voicing section), and things start to get dicey. If the treble controls are set too high, a high pitch whistle sets in. To eliminate the whistle, I need to either;

A) Ease back on the treble, or
B) Dial the voicing back up (for less voicing signal), or
C) Dial back the master volume from max.

Right now, if I add any voicing, I really have to roll back on the treble. With the treble maxed, or worse yet boosted, the voicing creates the whistle noise, so I have to back off the master volume. If I 0 the voicing (max input from the voicing circuit), and dime the treble, I can't get past 5 or 6 on the master volume without the whistle setting in.

It's as if there is some kind of interactive conflict between the preamps, and the voicing circuit. Especially if I use the treble boost. Could it be that too much treble frequency overloads the voicing circuit, causing the whistle? Is a combination of the preamp, then being boosted by the voicing circuit, messing with the power amp section? The whistle sound is very high pitched, and the guitar volume drops when it sets in. Using channel 1, if I dial up the drive, whistle then starts to really get loud.

I would like to know if it's just a nature of the beast, or if it's a curable problem. I can't see why this amp shouldn't be able to run flat out, no matter where the treble, or voicing controls, are set.

The complete schematics of the unit are here; http://www.aronnelson.com/gallery/main.php?g2_view=core.DownloadItem&g2_itemId=49938&g2_GALLERYSID=d5499ba2a6cd2a2d0c3f48e1c8b6e689

PLEASE NOTE THAT THIS UNIT'S PREAMP OPAMPS ARE THE 4558 VARIETY, NOT THE 739'S. BOTH VERSIONS ARE INCLUDED IN THE SCHEMATICS PACKAGE.

PRR, R.G., you guys were instrumental in helping me fix my Ampeg, can you spare some guidance here?

*UPDATE*: On channel 2 (the clean channel), with both preamp and master volume maxed, the voicing input at minimum, the whistle sets in when I get the treble control past 6 in the boosted mode. I'm beginning to doubt the power section here...

[wavley]

I have a Kustom 100, it's a little older than yours and mine is fully discrete and TERRIBLY noisy and harsh.  First, I lowered the value of the mixer resistors on the power amp board for the two channels, it helped a bit with noise.  What I ended up doing was disabling the non-vibrato channel that I never use and replacing all the npn's in the signal path up to and including the mixer amp with lower gain BC141's and I ended up with an amp that was quiet enough hiss wise to record with a little less volume (but there is still plenty on tap) and without the harsh treble.  Mine doesn't have the fuzz built in, so I'm guessing killing a channel is a bad thing for you.

But I digress, it looks like you have some sort of oscillation.  Have you checked the caps around the op amps on the voicing board?  Maybe something there to kill oscillation has a bad solder or something.  Do the treble controls on both channels effect the whistling?  Maybe, if the two channels are interacting too much upping the values of R7 and R8 might help.

[digi2t]

Maybe, if the two channels are interacting too much upping the values of R7 and R8 might help.

Hey... you might be on to something there...

I do indeed get interaction from the other treble control.

But shouldn't the fact that the unused input is grounded, kill any output from that preamp, or am I facing leakage here?

[digi2t]

OK, results of the new test.

Using channel 1, zero the volume on channel 2... bingo! No whistling. I can dime everything, full input from the voicing section, and it's solid.

Using channel 2, zero the volume on channel 1, whistling sets in when treble is boosted, and past 6 on the dial.

Both tests with the master volume at max.

Something might be awry with channel 2?

[Mark Hammer]

Dino neglected to mention it has been sitting on its end (i.e., resting vertically, with the controls in a column) for several years.  Maybe the electrons fell over?   

Can't see the schems at work, but I look forward to giving it the once or twice over tonight.

[wavley]

OK, results of the new test.

Using channel 1, zero the volume on channel 2... bingo! No whistling. I can dime everything, full input from the voicing section, and it's solid.

Using channel 2, zero the volume on channel 1, whistling sets in when treble is boosted, and past 6 on the dial.

Both tests with the master volume at max.

Something might be awry with channel 2?

Just curious, is your reverb on during this?  Channel 2 has verb and vibe right?  Try pulling the leads to the tank just to rule out strange spring interaction.

Also, channel one has a 150 pF cap on the input, two has 47 pF.  Looks like maybe there is a lot more bleeding treble/offending oscillation to ground on one vs. two

[armdnrdy]

I believe that because this amp was stored on it's side......it is now ruined. let me explain.

When you store an amp on it's side, the electrolyte settles away from the capacitors pins.
Over time the electrolyte will turn to a jelly like substance (not unlike your Aunt Mable's apricot jam) no longer making contact with the pins.

So in essence....you would have to recap the whole amp which would be very labor intensive as well as expensive for those large high voltage caps.

But I have a solution for you! You can send the amp to me for spare parts (I'll even pay for shipping) that way you can concentrate on all of the other stompbox projects you have going as well as projects around the home!

You will thank me, your wife will thank me, and your dog will thank me!

One more thing Dino, if you believed one word of this..........fish on! 

[Mark Hammer]

OK, results of the new test.

Using channel 1, zero the volume on channel 2... bingo! No whistling. I can dime everything, full input from the voicing section, and it's solid.

Using channel 2, zero the volume on channel 1, whistling sets in when treble is boosted, and past 6 on the dial.

Both tests with the master volume at max.

Something might be awry with channel 2?

Just curious, is your reverb on during this?  Channel 2 has verb and vibe right?  Try pulling the leads to the tank just to rule out strange spring interaction.

Also, channel one has a 150 pF cap on the input, two has 47 pF.  Looks like maybe there is a lot more bleeding treble/offending oscillation to ground on one vs. two

There are RCA jacks for the tank, but no tank.  It is a "reverb-ready" but not "reverb-equipped" amp.

[wavley]

OK, results of the new test.

Using channel 1, zero the volume on channel 2... bingo! No whistling. I can dime everything, full input from the voicing section, and it's solid.

Using channel 2, zero the volume on channel 1, whistling sets in when treble is boosted, and past 6 on the dial.

Both tests with the master volume at max.

Something might be awry with channel 2?

There are RCA jacks for the tank, but no tank.  It is a "reverb-ready" but not "reverb-equipped" amp.

Well, that rules out reverb funniness unless those leads want to be properly terminated to not oscillate.

Just curious, is your reverb on during this?  Channel 2 has verb and vibe right?  Try pulling the leads to the tank just to rule out strange spring interaction.

Also, channel one has a 150 pF cap on the input, two has 47 pF.  Looks like maybe there is a lot more bleeding treble/offending oscillation to ground on one vs. two

edit: moved my comment out of the quote so it would make sense

[digi2t]

Like I mentioned before, the head looks to be out of a combo, so I believe that the reverb tank ended up disappearing with the cab. Wiring and connections seem to look righteous enough inside the head.

I`m leaning to channel 2 being the offender here. Especially since my last test. Channel 1 really rips with channel 2 volume and treble at 0. No whistling whatsoever, even at max volume and no matter where I position the voicing pot. I`m going to have to pull the preamp board for channel 2 and take a gander at it.

Another thing I noticed is that there are two schematics for the channel 1 board (4558 and 739 opamps), but only one for the channel 2 board. I have to verify which opamp arrangement is represented in the schematics, 4558, or 739. If it`s the 739 board shown in the schematics, then I`ll have to ammend it to reflect the 4558 board that is in the unit.

Larry - I already thought about the caps being on their side. That`s why I was careful to set it on it`s opposite side for a full 24 hours before using it. You know... like a fridge. 

[Mark Hammer]

Larry - I already thought about the caps being on their side. That`s why I was careful to set it on it`s opposite side for a full 24 hours before using it. You know... like a fridge.  

You see, now THAT's the sort of insight that makes me trust your judgment.

Finally got to look at the schematic, and that Voicing thing is definitely a different sort of twist than one finds on most two channel amps.  Recognizing that sometimes things fail to make sense simply because of how they're drawn, I took the liberty to redraw part of the voicing circuit.  Here is what the 100k pot portion "actually" is:

When the wiper is moved fully to "the left", we have a 4k7 and 1500pf placed in parallel with a 1M input resistor, and an odd sort of feedback loop that has a 1M resistance in the "standard" position, but those additional components going from output not to the inverting input but to the junction of R10/C7.  If we rotate the wiper fully in the other direction, we have a 1500pf cap and 4k7 (in series) placed in parallel with 1M and the full pot resistance (100k), .01uf cap, and 4k7 (R10) fed back to the point before the input resistor (R11).  So, it would appear to have a kind of treble boost/cut function, but not standard.

The ganged 10k section would appear to adjust attenuation, conceivably to compensate for whatever the tonal adjustment does to signal level.

I take it the chip labelled 80848 in the schematic is a 3080, given the pinout, and especially given the 150k/4k7 attenuator on the input.  If you can scope the LFO output waveform, I'd be curious about its basic shape.

[R.G.]

I would first get it as close to stock as possible - which in this case means replacing all the electro caps. If they're original, they're in questionable condition.

If you can't find replacements for the two big ones, replace all the others.

Electro cap aging and going higher impedance can easily let signal leak between sections through the power supply and cause spurious oscillations.

[PRR]

> I took the liberty to redraw

Thank You Very Much.

My one cent:

Squealing with any treble-boost, in an amp with wires, suggests moving the wires around. I had a clone Champ squeal, putting a pencil between two wires made it worse, pushing those wires apart just a half inch made it quite stable.

Old e-Caps do what R.G. sez: don't work so good, and sometimes "only" at high audio frequencies.

And another thing R.G. has said before: don't discount the power of bad solder joints. Hasty-production with too-many parts/wires plus 37 years of age and abuse (and sideways gravity??), stuff goes bad.

[digi2t]

Yeah, I already thought about the caps, and it`s on the to-do list. The big ones are.... gulp... wow. I need to post a gut shot, they`re something! Looks almost hi-fi.

At work right now, so I can`t see Mark`s redraw. Check it when I get home in the morning.

This baby is worth the effort. It really has the potential to give my Ampeg a run for it`s money. Speaking of which, I was also contemplating upgrading the driver and power transistors in the power section, as I did on the Ampeg. Don`t have the number on what I used on the Ampeg off hand, but I`ll find them later, and run them by you guys for opinions.

[PRR]

Anybody know power amps?

This power-amp lacks any coil or R-C at the speaker terminals. "Most" basic transistor amps "require" these. I wonder how wild the supersonic response is?

Also note that the "protection" transistors only sense current, not voltage (or SOA). Further: the averaging network means that when Q10 fails open, Q11 is allowed *double* the total current (four times its nominal protection-current) and ain't going to last another loud note.

the bootstrapping to VAS Q3 is pretty bold stuff. Don't think I've seen that before. (Something 'like' that in many ICs, but without the cap jacking beyond the rails.)

It also appears, from the numbers, to be less than 100 Watts clean sine-wave (maybe 80W), but probably nobody said "sine" and digi2t is not saying lack-of-power. 130W real-squashed-wave is sure possible, and easy to "show" on an analog VOM. (This was about the time of the first portable DMM at a price a technician could "afford"--- else we used VTVMs.)

[PRR]

> The big ones are.... gulp... wow. I need to post a gut shot

4,500uFd 50V? Bah, that's not big anymore. 'Bout a 1-inch can. 3 bux.
Typically *about* --
Height 1.378" (35.00mm)
Dia 0.866" (22.00mm)

http://www.digikey.com/product-detail/en/ECO-S1HP472BA/P6931-ND/132134
http://www.digikey.com/product-detail/en/UVR1H472MRD6/493-1116-ND/588857
(You can pay 6 bux for "audio cap" "power supply filters for sound quality priority audio equipment" but I don't know why. Anyway welder-caps have better ripple specs, and if this amp is HARD-run that might be marginal.)
http://www.digikey.com/product-detail/en/LAO-50V472MPDS2A%23/604-1095-ND/2171146

[digi2t]

OK, found a quick gut shot of a unit on the net. It's the same as mine. Amply shows the two big power caps. Giants to me, but I haven't seen that much in my time...



Of course, like all modern components, it stands to reason that equivalents (or better), would be much smaller now.

I found the numbers on the drivers and power trannies that I used on the Ampeg; MJ15003's, with the Toshiba 2SA1837 / 2SC4793 combo. They should do the job in here as well, no? It's a fantastic combination in my Ampeg.

Would it be worthwhile to replace the 4558's while I'm in here? Would a TL or OPA perform better?

Off to inventory the caps in this baby. Come to Popa!

[Mark Hammer]

Not all THAT many electrolytics in there.  The ones in the power supply will pose a challenge for replacing, largely because any modern replacement will be about 1/5 the size and not fit in the harness provided.  That will certainly make it a lighter amp, but you'll still need to figure out how to secure them.  I'm sure you'll work up something clever.

BTW, you need to clip your toenails.

[digi2t]

Yeah, I'm not really worried about the big caps. I've done some research, and whatever caps I do use to replace them, vertical or horizontal mounting straps are widely available. It's just a matter of "out with the old, and in with the new".

The toes you see, belong my stunt double. I'll pass on the message.

[tubegeek]

BTW, you need to clip your toenails.

Soft clip or...?