Ampeg v4/v22 filter caps substitution question

[iandy4]

Hi Everyone,

I hope this question isn't too out of place in this forum.  I purchased an Ampeg v4 head with the two cabs a few months back.  The second time I played it, thirty minutes in it started pouring smoke from the input jacks on the front panel.  I opened it up an saw that the smoke was most likely all from C19 in the schematic below.  The other filter caps were bulging a bit as well so I ordered new electrolytic's for the filter section which as far as I can tell is C17, C18, C19, C20.  I ordered replacement caps for the whole amp actually but my question pertains to C17-C20.



Instead of getting the original can caps which are:
2 - 40uf x 100uf can caps (C18, C20)
1 - 40uf x 40uf x 40uf can cap (C17)
1 - 40uf axial cap (C19)

I ordered (all 500V):
1 - 50uf x 50uf can cap
1 - 100uf x 100uf can cap
3 - 47uf axial caps

The problem is that I assumed that in the new can caps I ordered, the individual caps within the can wouldn't share the negative terminal.  That means I can't wire the 100uf's in series like in the schematic.  That being the case I think I still have the possibility to wire this thing up in time for our gig on Saturday if I can find a 100uf or a close substitute. I have two other capacitors laying around.  1 - 80uf @450V and 1 - 270uf @450V.  

That gives me these values to replace C17 - C20:
1 - 50uf x 50uf @500V
1 - 100uf x 100uf @500V
3 - 47uf @500V
1 - 80uf @450V
1 - 270uf @450V

My question is: firstly, are either the 80uf and 270uf a safe substitute for one of the 100uf's in the schematic (C20 - C18) and if so, which would be better (it will be in series with the 100uf I ordered) If either cap is a viable substitute that means I can replace C17 with the 50x50uf can cap and one of the 47uf's I ordered.  The 40uf half of C18 and C19 can be replaced with the other two 47uf's I ordered. Leaving me with the 80uf, 270uf, 100uf half of new can cap to replace the two 100uf's in series formed by the halves of C18 and C20.  

If anyone has some advice on what my options are I'd really appreciate it.  If I can't fix it and test run it before Saturday I'll have to play a crappy little bass amp at this big music festival

[iandy4]

Anyone? I assume I should use the 270uf as a replacement for the 100uf.  I could use a confirmation for the peace of mind though.

[Derringer]

1.) there are disgustingly high voltages in that thing

I believe the rule of thumb is that so long as you observe the voltage ratings, you can go with larger caps in the PS

But I and probably most people here fear giving you definite advice because (see #1)

Now between the 80 and 270 uf can, I feel like going to the 80uf cap might be safer because the 270uF cap is SOOO large compared to what the amp is expecting to see ... but if it were me, I'd just gig with another amp and wait for the right parts.

Hopefully PRR will chime in. He'll know for sure.

[iandy4]

Thanks! That is exactly what I was thinking.  I believe the 100k's around the filter caps work to split the voltage between the caps and I'm not how changing one cap value of the two 100uf's would change matters.

[iandy4]

Anybody else?

[liquids]

ampegv4.com

[iandy4]

Understood. 

[liquids]

It's a great site...I've posted hundreds of times there, too, exclusively about the V4....

I am a bit sorry that I don't have the wind right now to disect your situation...and it's not that my response was to say that it's not welcome here....but that forum is devoted to that amp and it's kin! ...and also other ampegs, to a degree    

As aforementioned by someone else, these amps are a unique beast; there are countless stories on that forum of paid tube amp techs  making these worse in attempt to repair them, rather than better, since often they're only familiar with the common fender/marshall/vox engineered stuff etc...this one is quirky and quirky and quirky....engineered differently in many respects...

Search and sort through posts from 'hangman' (steve) and my posts (I am liquids there, too) and "steelyman" over there...there is plenty of other good and similar threads, pictures, specifics, how-tos...an amazing array of info available on this topic and this amp.   It invariably takes some time to sort it out...and no harm in posting a new thread there...but it's to help, not to push you away...just lead you somewhere full of info as it were.  No intent to offend with my previous ultra-brief response.

My ampeg V4 is my favorite amp - it made me sell  a long-used 1968 super reverb that was my #1 amp (which had no 'vintage' type value since someone had ripped out the tube chart, so it was a players amp), which saw some mods even... that is an amazing amp too! Best I had ever played....until I came across an ampeg V series amp.   Somehow, my V4 runs circles around it, IMO!  I fixed the V4(b) up myself via a tube tech friend's encouragement and the info I got from that forum and the internet etc.  Then the Super Reverb just sat there.  I sold it, sad a day as it was...it deserved to be played.  But my V4b would forever get all the attention, I knew it.    Now I have a beat reverberocket II I scored for cheap with some overtime cash I wanted to blow...that just sits there too, and that too is an amazing amp!   V4b is just hard not to play.

If I can find the hot air I am usually full of to type more about your particular issue, I'll by all means do so!   You've got a winner of an amp in hand!

[iandy4]

Thanks Liquids! I was aware of that forum before I decided to post about it here.  Although it's obviously more related, when I was searching through the old threads the posts seemed to be drawn out over weeks or months but admittedly I didn't look too long.  I realized that I shouldn't have posted this here when I didn't get a response because I know this forum better than any other and was kind of hoping for a super technical yet straight to the point answer from R.G. PPR or Processaurus that'd lead me in the right direction to learn about it.  I suppose if I got the answers it would just encourage me and others to post about other non-pedal related questions. 

I talked to a friend of a friends dad who is an electrical engineer on the phone last night.  He told me the 100uf's have to match.  I knew I should just wait for the right parts but so badly want to use this beast!   Oh well!

I hope I didn't upset anyone in the forum and I really appreciate you pointing me in the right direction.  Thanks man!

[amptramp]

I would suggest you check whether the caps are getting damaged for a reason.  Is there a shorted diode in the diode bridge rectifier?  Did you lose your -62 volt bias?  This amp has fixed bias and anything that reduces the bias voltage requires more current.  Maybe the diodes and transformers can take it but the caps cannot handle the extra ripple current.  Check that the 100K 2W resistors, R56 and R57 are close to equal - they set the current of 2.725 mA that balances the capacitor voltages.  If they stray too far from each other, this causes unbalanced voltages to appear across the capacitors and this can lead to failure.  C19 is in series with the 40 µF section of C18 so if either capacitor shorts, the other one is left with the full 535 volts across it and they have no balance resistors to split the load evenly.  One section of 100 will work in series with both sections in parallel of the 50 x 50 for the main filter at the rectifier output.  Individual 47 µF caps in series should work for the combination of C19 and the section of C18 in series with it.

If the capacitors don't match, the voltage split between them will go unbalanced as well.  If you have a 600 volt supply and there are two caps in series, a 50 µF and a 100 µF, 400 volts will appear across the 50 and 200 volts will appear across the 100 without the voltage splitting resistors.  For your specific question, the 80 and 270 capacitors should not be used in series with dissimilar values.  You might get away with the 80 (since most capacitors have a tolerance of over 20%) but if the tolerance is the wrong way, the voltage split will be bad and the hum will increase.  C17 replacements are not critical for capacitance - as long as the capacitor is equal or larger in value, the replacement will be OK.  The 450 volt caps will be marginal for C17.  If C17 is OK, leave it.  You can use the trick of double capacitors in series to replace individual sections of C17, but there may be ESR issues.

Be aware that before the tube heaters warm up, all filter caps except those in the bias circuit will see high voltage (over 600 volts) so NEVER turn on the unit unless the standby is off and do not turn standby on until the heaters are warmed up.

[PRR]

> wire this thing up in time for our gig on Saturday

I'm pretty brave but I would NOT rush into a VT-40 (same circuit), under any time constraint.

It's too stange, too tight, too dangerous.

Last time I procrastinated several years, finally got inspired, and then made a MAJOR blunder which was putting out DOUBLE the high voltage. (I'm brave BUT suspicious... I saw the B+ going WAY high and shut-down before the pulp hit the ceiling.)

> check whether the caps are getting damaged for a reason

If they are original, it's just time. They were not super-good caps to begin with, they are crowded in with large hot resistors, the amp runs very hot (especially in 4-bottle models).

I would say that loss of negative bias "usually" won't take out the power caps, but in a VT-40 dang near anything could happen. (My inspiration was to de-power from fix-bias 60W at 590V to self-bias 23W at 400V... it has much of the original tone with far less stress on parts, and is more suitable for smaller venues or with good PA.)

[liquids]

Cathode biasing is definitely something people have touted over at ampegv4.com as a good alternative/compromise to keeping things 'as designed;' some speak of it similar as having negligible losses or more benefits than losses....especially with the V2/VT-40 type seeing near 600v on the plates and screens.

PRR - didn't know you were a VT-40 owner up until now.   Nice!

What was your approach to reducing B+ outside of eliminating the negative bias on the grids and cathode biasing (etc. in that area)?  
Did you get 400v on the plates via new power transformer, another approach, etc?

I have an old silverface bandmaster head around, mostly gutted and was using for expiriments.  I've considered and mentioned even on the forum there, that I might use it as a means to diy a v-series preamp (via a trashed V4 I got for hard to find parts), utilizing the PCBs or maybe only the multi-tap midrange inductor from the V series or PTP the circuit, have some chassis-mounted preamp tube sockets, means to eliminate the 6K11 and use the bandmasters transformers (or maybe replacement transformers of some ilk) to power and output 6L6s at a lower voltage or other tubes for a power amp with a V-series type preamp.  It's way off on the project list, but it wasn't really an idea I mentioned outloud that was well taken over there.  

The forums is no less a good rescource, and I like the back and forth, even if some cry 'heresy' over an approach, at times.

[liquids]

> wire this thing up in time for our gig on Saturday

I'm pretty brave but I would NOT rush into a VT-40 (same circuit), under any time constraint.

It's too stange, too tight, too dangerous.

Agreed on time constraints, strangeness, and danger - and said by someone much more respected than I am (and rightly so, no doubt).  I am however, not pretty brave, as you indicated you are; it's good to know it's not just my own 'lack of bravery' that would keep me from doing so or suggesting that...I all but hit 'send' on a reply suggesting that as great as the amp may be, the time constraints are too confining, as one who's done only a bit of amp work, but has poked inside a V-series Ampeg...

Firing the amp up 'open' on the bench the first time alone made me wonder if I was pushing my bravery or had entered lunacy...put on goggles(!), connected it to a speaker load, closed my eyes, said a short prayer, plugged it in, flicked the power switch...waited...took it off standby....

Well, somehow, I am still here!

[PRR]

> you were a VT-40 owner

I worked for a place which owned one since new.

It was reported "dead", I diagnosed an ill PT, autopsy showed a BIG burn/melt event inside the bell-cover.

(It also had a bad 7027, but I did not discover that until later.)

Nobody remembered how it happened.

That was before the Internet. It was not so easy to find parts; also money was tight (slim repair budget and horrid bureaucracy).

> What was your approach to reducing B+

Some years later I was surfing the web, and realized that a generic 5F6a PT was very affordable, practically pocket change. That gives ~~400V, less insane than 590V. With the lower B+ and the VT-40's higher OT impedance (6.6K instead of 4K), power would be way down even from 5F6A, but a stock VT-40 is way too loud for anybody past 30. With the four low-mass 10" speakers in a VT-40, I felt sure it would be useful even at a fraction of original power.

Also the lower B+ might not blow the elderly caps so soon.

Further calcs said it could run self-bias near Class A, and with substantial screen resistor. This meant that NEXT time a tube went bad, it would probably burn a power resistor instead of a transformer. Making that diagnosis and repair much easier.

One glitch: my model used FWB rectifier, the 5F6a PT is for a 2-diode rect. I overlooked that, red to red, and at switch-on was headed for 800V! I was glad I _always_ have a B+ meter on an amp. Snip-snip the extra diodes, 395V.

250r 10W common cathode resistor. ~~3K common screen resistor-- too high for best sustained Power, gives some flavor to attack and limits damage in an accident.

470K from pin 1 to cathode. This allows common EL34 to bias correctly, while not smoking if 7027 are found and used. Can also run 6550, 6L6GC and friends, even metal 6L6 and probably modern 6V6. On the 'scope it works the same with two new EL34 or with one vintage 6550 and one metal 6L6.

This model has a Fender-like preamp with a trace of color and a "Distortion" preamp with either NO color or FUZZ! color. Even my tin ears could tell the difference, squeaky-clean or naked-tube.

IMHO, the main fault of these V-series amps, after the insane voltages and ugly cap arrays, is that PCB. Aside from maintainability, it ensures hummmm. Not so much in the VT-40: the amp output is riddled with hum but the open-back four-10" speaker won't reproduce 60Hz more than a foot away (it cancels-out). On a better-bass speaker it might be worth slashing the heater traces and running over-board twisted-pair heater power.

[liquids]

Only thing that I would say is...I've not used a combo V-series, nor or a distortion model V-series....

however...when running well, most people report that they are surprisingly quiet and incredibly loud at the same time....

Not certain of why yours hums, but mine is PCB too, and is a church mouse.  

The PCBs do seem like they got flimsier and the traces less solid as time went on, even through the time from first serial number up to last non-distortion serial number

The distortion model may well have a VERY different wiring and PCB layout than the non-distortion models.  

So for me it is hard to compare.  

I personally don't think that noise/hummm all comes down to it being PCB vs not PCB as a generalization - though you may not have been suggesting such a broad brush generalization yourself; it may be that particular PCB, wiring, etc. Can't say I know what you think or what it is for certain...just hoping no one shys away from giving one a chance thinking they're all noisy.  Far from it.

[liquids]

Kinda 'jumping' this thread, like a car that's sat for 4 months...

My V4 runs fine, but I tinker with it from time to time when so inspired.
One concern I have, related to the B+ that gets up to ~540V depending on the wall voltage, is that the screen voltage (B+2) is within 1-2v of that at idle.  
540v is pretty high for most 6L6GC plates, but especially high for most 6L6GC screens. I've got cheap tubes and rugged tubes that can handle it, but it's not ideal in my mind.

Per the schem/stock, the screen was fed via a tap off the V+ through a 470E/7W filtered via a working capacitance of 20uF to ground, and then each tube's screen has it's own 470E/5W screen resistor.
In my particular amp this network is replaced by a 500E/10W, filtered by a 25uF/800v cap, and each tube's screen has it's own 1k/5W resistor.

Anyhow, I've wanted to drop screen voltage. Likewise, I've wanted to beef up the screen filter supply.

I'm okay with tinkering with modding part values in the circuit and such, but I'm not big on drilling-type mods (like add a choke) - I'd rather do a semi-scratch build for that, based on the V-series Ampegs (some day), if I went that far.

I tinkered with feeding the screens from various arrangements of cap-bypassed voltage dividers with resistance values that effectively allow for the screens to see lower voltage, but >50% of the B+ voltage.  I liked the sound of about 80% voltage on the screens (didn't go much higher than that), but due to time, parts, and lack of knowledge, I couldn't effectively isolate the variables enough to know if it was the screen voltage difference that I proffered in the A/B comparison, or due to others.  Also, I've found that my approach, practically speaking, is 'less than ideal' for the effective resistance the screens are seeing at DC.

Some parts have arrived now, and others are on the way.  At the moment,  I am intending to replace the screen filter capacitance with 2x100uF/450V caps in series for an effective capacitance of 50uF, to stiffen the supply.  It seems best to parallel resistors with the caps because of the cap's voltage ratings, to start, though, it's not always done....

It seems to me, as I've thought it over, that I can put high power, lower value resistors in parallel with the cap, and that would reduce the voltage, as an alternative to 220k+ value resistors in series across those caps.  

If I use smaller resistance values, the voltage should drop, but screens are not seeing increased resistance, in series;   a voltage divider is being created with the 500E/10W resistor that taps off the B+.  

From the scenarios I've calculated, this will drop voltage up to 10% depending upon the arrangement I settle in on.  It will get the screen voltage a little lower, and maybe more into the safety zone for any pair/quad of 6L6GC -even the most rugged .  It will also,  potentially be a bit more optimal for a pair of KT88s I intend to experiment with using (a pair, not a quad).

I've done some math, and I have a pair of 25k/5W resistors that can be a more 'subtle' voltage divider, which is probably where I'll start.

I  also ordered a handful of 10k/20W resistors.  I can configure these to effectively work as a 20k, or 10k of resistance to ground by putting two in series with the two caps in series, or two paralleled pairs in series.  This will create a more notable voltage divider in conjunction with the stock 500E/10W series resistance, up to 1K/10W, which is about where I'd max out unless told otherwise.

That said – I suspect that this much added 'load' for the rectified DC B+ voltage can effectively drop the overall B+, and/or increase ripple, in practice, once I get down into that zone of load.  Of course, I understand that it will also add heat that will likely be significant, if not with the pair totaling 50k, than definitely with an arrangement that totals 20k or 10k.

I'll have to see how much and where the cost/benefit ratio starts to come out as disadvantageous for me in practice, over keeping the current arrangement of 25uF, or keeping the 50uF arrangement but using large value resistors to minimize the voltage drop to something more minimalistic.

KT88 tubes seem to beg for 1k resistors in series with the screen. 6L6GC tubes apparently draw low screen current (comparatively, and when pushed into clipping), and the classic screen resistor value for 6L6GC tubes is 470E, but I've had 1K resistors in there regardless...

If I like the voltage divider scheme, one way to retain lowish resistance in series with the screens but still drop voltage (or do so even more) would be to increase what is now the 500E/10W resistor significantly, up to and past 1k (say, to 1.5k or 2K), but reduce the individual screen resistor values to 500E or even 100E.   So, am I asking for trouble with screens of the KT88s in particular seeing 2k+ of total resistance in series between it and B+, filtered, but the individual screen resistors being lower than 1k – such as 500E, 100E?

Any other thoughts?

[amptramp]

The standard design centre values for line voltage would have been 115 or 117 when these amps were designed.  Now that voltages over 120 are common, maybe a small transformer with the primary across the input voltage and the secondary in series with the input, bucking the input voltage would also be a good idea.  The voltages you are running at are based on the lower input voltage compared to the 121 volts we have where I am.  A 6.3 volt filament transformer with the output rated for the input current of the entire amplifier would be a reasonable modification because it does not add to the dissipation like extra resistors.  Your plate, screen and filament voltages will all come down to where they were originally designed.  Your 540 VDC supply would come down to 512 VDC - maybe not a big improvement but certainly a step in the right direction.

I also support the idea of dropping the screen voltage by means of the divider network you have described.

[R.G.]

See "The Vintage Voltage Adapter" at geofex for how to wire up a filament trannie as a bucking transformer.
http://geofex.com/Article_Folders/vintvolt/vintvolt.htm

These can be quite small and compact. Here's one I did for a friend's Deluxe:


Do NOT try this unless you already know how to wire it up safely, either inside your tube amp, or in a separate metal box, as above.

[PRR]

> ~540V .... the screen voltage (B+2) is within 1-2v of that at idle.

Point to a schematic. please.

There were several things marketed as "VT40". Some have a *separate* winding to make G2 power. Nominally 338V.

Taking G2 from >500V plate supply with 6.6K load is reckless.

EDIT: I found a plan noted as "1971" which does take G2 from 595V through a K of resistor. Ampeg was hurting to sell amps against the Fender steamroller.

There's not a good answer. Using EL34, any sufficiently low-resistance voltage-divider is gonna eat near as much power as the plates. I have used MOSFET dropper but it's tricky. You could rig a 6V:230V transformer to make some +350V, a good number. All of this goes against the tight tangled chassis.

[liquids]

My amp is the classic, early-circuit style V-4b (if there were even any other variations on the V-4b model).  

I'm sorry that I didn't post a schematic.  Here:  http://www.ampegv4.com/images/schematics/V4B.jpg

As you can see in the linked schematic, it has voltages, and 540V/545V is expected.  The V-4/V-4b generally all had 540V B+ on the schematic.  It's not a line voltage issue.  The residence where I am currently plugging it into gets ~116V AC at the outlet most of the time, and I get about 516-520v DC on the B+.  I've gotten 540V in other residential areas.  I've never measured it in commercial areas, where indeed it may go higher, but I think 540V B+ is assuming 120v AC at the wall.

Later models of at least some V-series amps had a separate winding for a lower screen voltage, yes - but according to the schematics I have seen, these have the same B+ voltage on the plates; it is as if 6550s were the design goal, even though 7027s are called for, as I see it.  

V-series ampegs had a schematic glued to the chassis cover - hopefully the right version of the schematic for the circuit as sent out from the factory.  

The V-2 amps - which are effectively the same circuit as the equivalent period V-4 but with only 2 power tubes and different transformers - often indicate 594V B+ on the schematics.    

VT-40s are a combo version of the V-2 with 4x10s.  VT-22s are a combo version of the V4, with 2x12s.

My chassis is not tight and tangled.

6L6GC tubes survive in my amp, basically.  The cold bias, for one, probably helps, comes stock, is the way I ended up leaving it after tweaking with it, and sounds best to me in this particular amp.

Tube survival is not the issue...it's more an interest thing.  I'm interested in the sound of screen voltage NOT being the same voltage as the plates, as measured for all practical purposes, the way it is in most amps.  This amp is, foremost, the main amp I actually use, and the amp that could probably benefit most from dropping screen voltage.  So a little tweaking of the screen voltage - especially for the sake of SED 6L6GCs, which sound great and hold up to the amp, stock, but are expensive - is intriguing, if not, an interest...

I can work on my own amps.  I've worked on this amp since I got it, exclusively.  I'm still alive, and do ever aim to be extra careful and be 'respectful' of the electricity.

I don't like EL34s.  I bought KT88s which should do just fine with the voltages as an alternative.  

I like the amp more than any other amp, ever, as is.  I just also like to experiment...

Simultaneously, after a certain point of experimentation, I'd rather abandon tweaking an existing amp/circuit, and leave further experimentation for a scratch build of some ilk based on the V-series, someday, because - back to the previous point - I like the amp, as is, more than any other amp, ever.

Right now...three ideas on the front burner:  Check out stiffening the screen supply, Try out a pair of KT88s, and drop the screen voltage..via screen resistance in series with the screens as combined with resistance to ground in parallel with the screen filter capacitance.