B+ fuse blown on VTL ST-150

FullRangeMan · « **Reply #20 on:** 12 Apr 2013, 06:05 pm »

Quote from: Maritan on 12 Apr 2013, 12:21 am

Here's a pic of it:

I know the picture isn't the best, but not exactly white, but some of the chrome top (which I assume is the getter) is now missing, and a large part of what's remaining is blackish.

Does the chrome top turning white indicate anything specific?

Looks a portion of this getter is missing, death tube.

medium jim · « **Reply #21 on:** 12 Apr 2013, 06:21 pm »

Quote from: Ericus Rex on 12 Apr 2013, 12:55 am

The getter turning white means a new tube has lost its vacuum. Not exactly sure what a darkened getter means.

When a tube arch's or burns too hot it will turn the flashing dark and can even burn part of it off.

Jim

AllynW · « **Reply #22 on:** 13 Apr 2013, 06:34 pm »

Martin, I owned VTL 300 Deluxe Mono Blocks for many years. After a year of ownership, I had VTL add B+ and 10 amp fuses installed after a 6550 nuked some internal components. All tube faults after the fuses were installed were limited to blown fuses. I had those amps 12 years. For fuses I used Ebay and web electronics stores.

Good Luck

SteveFord · « **Reply #23 on:** 13 Apr 2013, 09:27 pm »

I've got 300 Deluxe mono blocks going right this minute, they're just great amps.
They came out with some sort of upgrade package /modification about a year after they were introduced due to what AllynW experienced.
If I were ever to change them out it would follow in his footsteps with the MB450s.

Maritan · « **Reply #24 on:** 13 Apr 2013, 11:14 pm »

AllynW - Are you saying that after you had fuses installed, when a tube blew, nothing else went bad? That's good to know!

I still am going to change the bias and screen resistors because they're pretty easy to get to. And this will make Steve happy - I figured out how to get to the bias pots. And what values they are. I'm going to order more precise 4 turn trim pots - the 3339 series "P" mount - and put them in there instead of the single turn 3386 pots.

By the way, my amp is only 10 years old.

I have a good soldering station, leaded solder, flux, cleaner set up. I have the new quad of tubes with me. I hope to get the fuses early next week. I'll be ordering the parts soon.

Still, I think I'll be sans music for another couple weeks.

SteveFord · « **Reply #25 on:** 13 Apr 2013, 11:46 pm »

New bias pots are cheap insurance and should hold you for another 10 years.
You might as well do it while the amp is down for a little bit and then it should be smooth sailing, finally!

AllynW · « **Reply #26 on:** 14 Apr 2013, 03:23 pm »

Martin,

When I purchased the VTL 300 Deluxe Mono Block (used) they came with the original VTL branded 6550A power tubes. One of the tubes arched and made a loud BANG. I took the amps to VTL and had them completely upgraded, which included: signature transformers, new caps, 10 amp and B+ fuse holders. When the amps were completed they could receive 6550 "C" type tubes. All of the above took place in the early part of this century. Over the years, I had multiple tube related "events" and it was always a simple and well practiced process to change the tube, b+ or 10 amp fuse. The fuses always did their job and I never had any other damage. FWIW: I've used GL KT-88's for the last few years and they do not have the longevity of the 6550 C tube.

The upgrade to the 450MKII: tube goes, amp shuts down. Blinking light identifies offender. Change tube! How sweet it is.

Maritan · « **Reply #27 on:** 21 Apr 2013, 05:18 pm »

Allyn, thanks for your detailed response.

Quick update - nothing has happened yet on the amp front, but that's because I'm waiting for parts, I got all the resistors I need, but am waiting on the trim pots. I decided to get both the OEM (3386 series 10k) and the 4 turn slightly higher precision 10k pot (3339 series). I'll try to put the higher precision pot in first and make sure I can access it from above/ outside. If I can, I'm going to preemptively replace all the older pots. I'm hoping they be here before next weekend so I can get the amp running soon.

I have been without well reproduced music for a bit too long. #FirstWorldProblems

SteveFord · « **Reply #28 on:** 21 Apr 2013, 08:53 pm »

That would be a good thing.
When my MB250s were down I plugged in a high powered SS amp for fill in duty and the following morning I lucked upon a pair of 300 deLuxes.
The seller was nice enough to take pity on me and the next night they were mine.
It had a tube socket with no power due to a cold solder joint or three but once that was taken care, new Winged Cs and some Supra LoRad power cords of it's been good ever since.
The sound quality is just addictive. Here's hoping your parts show up early next week and your troubles will be behind you.

Maritan · « **Reply #29 on:** 6 May 2013, 04:26 pm »

#FirstWorldProblems seemed to be solved as of last night.

Cliff notes version: Changed (and upgraded) trimpots, and swapped out new bias and screen resistors, and put in new fuses. Everything seems functional.

Long version: I had some family medical issues so I wasn't able to get to the amp as soon as I wanted to, but once the medical issues were cleared up (thankfully in a positive manner), I had time yesterday to work on my amp.

First up was the potentiometers (Bourns 3386) that Steve likes to bag on - and deservedly so. Unfortunately, there really aren't other options as far as manufacturers go for these things. What I did find though, was that I could get Bourns trimpots with more turns for a given range. See picture below for three versions of a 10k Ohm trimpot:

From left to right - 1 turn (actually ~280 deg and what comes with the amp), 4 turn, and 21 turn.

I sought and found each variant with the same footprint, but as you can see the packaging varies wildly. The 21 turn was going to be way overkill - I don't want to sit there and turn the pot for an eternity to get the biasing right. I also didn't like the packaging and wasn't sure if that tiny screw would be easily accessible off to one side of the package via the access port on the top of the amp.

I did want some additional precision, so I went with the 4 turn trimpot (the one in the middle in the picture above). They are slightly taller than the original trimpots, but not enough to cause any issues. Once mounted they sit right in the centre of the access port. They take a little more time to bias the tubes, but I can set the bias to 278mV if I want instead of 275mV. That's nice. What's not nice is that they don't have a defined stop (this is a problem with any multiturn pot), but once you have it biased you shouldn't be playing at the ends of the pot's range anyway, so it shouldn't be a problem.

Once all 8 trimpots were changed, came the slightly easier part (but still very time consuming) - changing out 8 bias resistors (Ohmite 10 Ohm 5W wirewound resistors) and 8 screen resistors (Ohmite 1K Ohm 5W wirewound resistors). The new bias resistors all measured around 10.1 ohms where the ones that were replaced measured between 10.5 and 10.6 ohms.

I then cleaned up all the solder joints I worked on, and some of the other solder joints that looked particularly dirty. I did go through the amp as much as I could and checked for (and didn't find) any cold solder joints.

Finally, I put two new B+ fuses in.

I put the amp back together and fired her up last night around 10 pm. She switched on, didn't blow a tube and biased well. And made glorious music.

I'm back baby!

SteveFord · « **Reply #30 on:** 6 May 2013, 04:35 pm »

A happy ending on all fronts is a good thing.

medium jim · « **Reply #31 on:** 6 May 2013, 07:36 pm »

One of the beauties of tube amps is that they're simple in design and that makes them serviceable. SS amps are just the opposite in many instances....

Jim

SteveFord · « **Reply #32 on:** 6 May 2013, 10:27 pm »

Maritan,
Where did you source the bias pots from?

Maritan · « **Reply #33 on:** 6 May 2013, 11:29 pm »

Steve - Just so you know, I ended up getting 10 of the OEM trimpots (that I did not use) from Digikey first.

I could have bought the 3339 series (4 turn) also from Digikey (at a little over $5 a piece), but since I had the OEM trimpots in hand, I decided to take a chance and ordered on eBay - $10 for 10 trimpots of the 3339 series shipped. Great price obviously, but I wasn't sure if I'd end up with a bunch of reject bin parts. I checked them with my multimeter and they all measured ok...

Jim - You're absolutely right. This was quite easy to work on. I didn't have the inclination this time, but if I absolutely had to, I could have traced the board out and drawn out a schematic. I did however note the values of all the capacitors in use, and I'm sorely tempted to start pricing them out and change them out when I have some time...

Ericus Rex · « **Reply #34 on:** 7 May 2013, 12:05 pm »

I'd say you made the right choice in those trim pots. I've got the square ones (on the right in your pic) in my amp and they are a little tricky to engage. My amp came with a special electronics screwdriver where the flat is actually inside a small cylinder which perfectly fits around the brass screw of the trimpot. The screwdriver is also insulated so slips won't send me to my grave. But I absolutely would not recommend sticking a regular screwdriver probing around the inside of an amp trying to adjust bias with these. What you picked is much safe and far easier to use.

Now, ROCK ON!

Maritan · « **Reply #35 on:** 7 May 2013, 03:46 pm »

Interesting. Is it the Quicksilver or the Rogue that came with the 21 turn trimpots and the special screwdriver? That special screwdriver would be a nice tool to have anyway.

I already have an insulated screwdriver to adjust the pots, so adjusting that trimpot with the brass screw wouldn't have been a problem as far as zapping myself (which is definitely a

), but the access to the screw would have been irritating at best... That was the main reason I nixed it.

And you are right sir - ROCK ON, indeed!

Ericus Rex · « **Reply #36 on:** 7 May 2013, 05:03 pm »

The Rogue. The Quickie is self biasing.

I did have the same Bourns as you (the 280 deg) in my Music Reference RM9 and had to replace all of them due to noise and non-functionality. I just don't understand why they're so fragile.

funnytube · « **Reply #37 on:** 30 Jun 2017, 09:07 pm »

Sorry to bring up an old thread, but this is the most recent thread discussing VTL's ST-150 repair.

A customer brought in a dead ST-150. This appears to be the early version, with horizontal slots on the front face, not a window. Testing the tubes showed the two 12BH7's had shorted. Inspecting components, one of the 12BH7 plate resistors (150K) had opened. Found a few cold solder joints. Fixed those issues, checked transformer DCR's, resistors, etc. I have David Manley's cookbook (which helps with basic topology), but no schematic (apparently unobtainium).

Variac power up. Hmm. Half the long-tailed pair phase splitter isn't working- no signal to half the push pull, on both channels. Start tracing the circuit out. Much to my dismay, it appears that the splitter is not wired for 12BH7's (9A pinout)- the grids and cathodes are swapped.

Digging through tube books, it looks like the 9CZ pinout would work. Tubes using this pinout are 12G8, 6350, 6463 and the 6840. Looking at the specs, it appears the 6350 is similar to the 12BH7. The 6463 is not too far off, either. The 6840 looks like it's mu is a fair bit higher. The only ref I can find is a mention of the 6350 tube for VTL amplifiers on a tube dealer's website (TubeWorld).

Can anyone shed light on the tube compliment for the early VTL ST-150? Thanks. -Kent, VintageElectric Services.

sfox7076 · « **Reply #38 on:** 1 Jul 2017, 01:21 am »

They used 6350s on the early ST-150s. They moved on to another tube when 6350s started to become scarce.

Shawn

funnytube · « **Reply #39 on:** 1 Jul 2017, 01:40 am »

Thanks. Got the amp running. Glad the 12BH7's didn't damage the unit.