Ampeg VT22 V4

[shortfuse]

I had acquired this amp a while back and never had time to mess with it.  It is a 1970 model and did not work when I got it.  Tonight I looked at it and first thing I notice is the power cord had been replaced with a 3 prong.  But the neutral white wire was running through the fuse not the black.  That is not the only problem I found but the hot should be the side fused correct?
I pulled the fuse and plugged it in and had 110v.  I then pulled out the lamp limiter and put the fuse in turned it on and the light lit up and stayed bright and the power pilot light would not come on.  Checked the power at the ends of the power switch and only had 49v.  So I got to thinking about what was going on and decided cant be the B+ side as I never turned on the standby.
I decide to remove the heater wires from the PT to the 1st power tube ormbegining of the heater wires and turned it back on lamp limiter dim.  All 9 pin tubes glowing and the pilot lamp is on now.  I plug in power tubes one at a time and the lamp gets brighter and stays that way every time I plug a power tube in.  And if I plug in more than one it gets even brighter.
Is it possible to blow all 4 tubes at the same time or did it blow something else first that has now affected the heater circuit and took the tubes out?  But the heater circuit is only to the heater filaments correct?  Or does the hot (black wire) not being fused have something to do with this?
Well I am surprised I got as far as I did to isolate where the problem lies.  More diagnostic time tomorrow.
I am going to take the tubes to work with me tomorrow there is a big tube shop around the corner from my office and he checks them for free.  Suggestions?

[sluckey]

I decide to remove the heater wires from the PT to the 1st power tube ormbegining of the heater wires and turned it back on lamp limiter dim.  All 9 pin tubes glowing and the pilot lamp is on now.  I plug in power tubes one at a time and the lamp gets brighter and stays that way every time I plug a power tube in.  And if I plug in more than one it gets even brighter.

But, was the lamp ever anywhere near full brightness? If not, you haven't really discovered anything yet.

The lamp should get brighter as you plug more tubes in. The additional filament current causes more current to flow thru the lamp. What you described is normal operation. And the pilot lamp is a neon that requires a certain voltage to fire, approx 85v IIRC, so that's not really a clue.

Leave the amp in standby mode and plug it straight into the wall. Be ready to unplug it quickly at the first sign of trouble. Do the tube filaments light up properly? Any smoke, smell, arc flashes, blown fuses?

[eleventeen]

The fuse in the wrong leg of the AC input is wrong; eg, bad practice, but should not affect the operation of the amp. You'll of course fix that.

You have a mystery situation (duh) where you're probably going to have to inspect the PC board and see if there is or is not a carbonized bridge between traces.

Your conclusion as to "it can't be the B+" is not necessarily correct as it appears that the big caps are charged (meaning, connected to the rectifier and the PT) even in stby.

I've looked at but never delved too deeply into these. I've seen some of them pretty beat up. They use high voltages (535 VDC) so note that. Also inspect the dropping resistors (5K, 5K, 2.2K) in the power supply, the ones that drop B+ for the preamp section. And the pads around them. As I recall, even though the chassis were shock-mounted, the trannies were quite heavy so that G-forces from handling and trucking these around seemed to inflict their evil effects on the amp.

[The_Gaz]

I've looked at but never delved too deeply into these. I've seen some of them pretty beat up. They use high voltages (535 VDC) so note that.

I know you're likely mentioning the high voltages to the OP for safety reasons, but it's not true that all V4's have those high voltages. To the OP: Make sure you have the correct schematic, or the voltages make send you on the wrong path while trouble shooting. It happened to me after dealing with a few of the HV units.

[PRR]

> white wire was running through the fuse

This is not wrong. Leave it be.

I don't know what you have proved. "Did not work" can be as simple as a bad jack or dud tube. When returning to an old idle amp, do lamp-limit it on first plug-in, but if the lamp-limiter does not go WAY bright you may as well just plug it in straight. Study the standby switch, these break. Now *observe*. Is it really totally dead, not a trace of hiss/hum in the speaker? Is the speaker plugged-in? Does the jack feel good? Does the speaker work on another amp? If it does hiss a little, do any knobs affect it? Plink the tubes.... if PI and tone tubes dink but nothing at preamp bottle, may just be one dead tube, or broken pot-lead or many things.

[shortfuse]

Well PRR after reading the posts I went with yours & Sluckey's suggestion I'm guessing I was about to way over think it again .  I plugged the amp straight into the wall and flipped the stby switch on and everything fired up ok all heaters glowing and no signs of smoke or bad smells.  There was a pretty good amount of humm but after a few minutes the 2 power tubes closest to the PT started to run away and glow very red so I shut it down.  I did take the tubes to work with me and had them checked at "Vacuum Tubes Inc" which is a 1/2 a block away from my office.  Man that guy has a lot of old tubes new in boxes and he is like a walking encyclopedia on tubes, ok enough of that but if you are ever looking for NOS tubes he probably has it.  Anyway 2 of them were toast and 2 were at minimal specs.  So we marked the tubes which were bad and when I got home Friday night I popped the 2 marginal tubes in the inside position V6 & V7 out of V5, V6, V7, V8 utilizing half of each side and played it with a guitar for about 40min. It played with no issues except that all the switches and pots could use a good cleaning.  I have only checked the power section as far as voltages and resistor values are concerned Resistors R40, R45, R39, R46 all measure about 4.6Ω the schematic says 10/5w, Grid resistors R37, R47, 38, R34all checked out at the 47K and R46, R41, R42, R47 at 470Ω.  With the Voltage readings attached in the pdf should I replace the plate resistors with 10Ω 5 Watt as it was when it was new.  Resistors R40, R45, R39, R46 are stamped 59300410 / PR-1.
I am confused as there is no cathode resistor, pin 8 on all tubes tie together and go straight to ground?  Or am I missing something?

As for tubes to get US made 7027A they are 55.00ea 220.00 for all 4.  I can get all 4 in JJ for 80.00 and they claim theirs are legit 7027A's.  Any other suggestions as far as tubes are concerned.  I would have to drop plate voltages to convert to 6L6GC wouldn't I?

[plexi50]

Just for fun i would change those old bias caps and resistor just to know they are not going to fail and ruin a new set of tubes. Those Ampegs get fairly hot inside the chassis and all the components take a heat bath over several years or even decades. Great amps/

[PRR]

Study Pin 1 on the output tubes' sockets.

_IF_ Pin 1 connects to Pin 4, you are stuck with 7027 until you modify the wiring.

If there is NOTHING connected:

Put in good-grade 6L6GC, done for now. (Don't worry about 545V on a "500V" tube.)

Put in good 6550, done for decades. In some Ampegs 6550 was a preferred option, recommended when the customer was killing 7027s. 

The 7027 was a 6L6GC-type carcass but with an extra Screen Grid connection on Pin 1. The datasheet suggests this adds cooling. Many-many amps don't connect to Pin 1 and work fine.

> there is no cathode resistor

It is a Fixed Bias amp. Bias comes from a negative supply around D6 R60.

I _do_ think it wise to snip the cathode wires and insert 1 ohm resistors so you can measure idle current safely.

[shortfuse]

Dave
Thanks for the advice I was thinking the same thing, I am already in there so might as well make that section new and worry free.  And thanks for the advice on the 2 caps also I did not take them into account.  This is an original 1970 amp never touched internally. 
Do I replace them with whats on the schematic 10Ω/5w or with a 5Ω/5w.  They all measure 4.6 to 4.9 right now.  They are stamped 59300410 / PR-1.  Does PR-1 stand for 10Ω?

PRR
Nothing on pin 1 so 6L6GC are much easier to come by. 
As all the cathode wires tie together from tube to tube and only 1 wire runs from the last tube to the board could I run each to a 4 lug tag strip and the tie them all back to that 1 wire?  Should they be done individually as in attached pdf sketch #1 or as a pair in #2?
Back to your earlier comment
"white wire was running through the fuse"

This is not wrong. Leave it be.

I thought you always wanted to fuse the hot leg?  Not the neutral?  Could you please explain further.
Thanks for the help.

[PRR]

#1

[thermion]

Put in good 6550, done for decades. In some Ampegs 6550 was a preferred option

I heard this too, that this was the hi-performance upgrade for these amps.
What I wonder is, since the V4 has screens at ~535vdc and the screen Vmax for 6550 is purported to be ~400vdc on some datasheets, will this duty promote premature screen failure in the 6550?
Would kt88 be a better drop-in, with its G2 Vmax around 600vdc?
Thanks!

[PRR]

> the hi-performance upgrade

High reliability.

> screen Vmax for 6550 is purported to be ~400vdc

440V measured.

But I don't take that too seriously.

6550 is for-sure a more robust tube than 7027/6L6GC.

AFAIK, KT88 is normally 6550 guts in a taller bottle. Yes, the ratings are different: Tung-Sol was known to be very conservative, Philips was known to push the envelope (EL34 at 800V? On an octal socket??). Like Ford rating the 289 2bb at 200HP (160+ honestly) and Dodge saying the 426 Hemi was 425HP (surely over 500HP).