2008 410H JVM Amp Problem

[dan_tramble]

HI

I want to ask advice of the experts here. I own a 2008 JVM 410 H model amplifier.

I bought it second hand. The one thing I might mention is when I bought it it did work OK however looking at the power tubes going from the vents looking down PI-1-2-3-4 then the PT ok 1-2 were JJ EL34 and 3-4 were Winged C The powertubes were mismatched.

Seeing this,

I initially retubed it with JJ EL34L power tubes and TAD preamp tubes with a JJ ECC803s balanced Phase Inverter tube.

I installed a 3 henries 250 mA choke as well. I put the exact same choke in my JCM 2000 DSL 100 and its been fine for a long time. I liked what that did to the amplifier and wanted a similar reaction in this amp. I read that the spec 3 henries 250 mA is fine for the JVM It is a Hammond brand choke.

I biased the amplifier with a dual bias probe to 65-70% disapation based off 480 VDC plate voltage.

I took the amplifier to band practice and played it at "band volume" and the volume dropped out. Looking from the vent going 1-2-3-4 from the PI tube #3 took off and went RED I shut the amplifier down, let it sit a touch and worked fine afterwards.

I took the amplifier home to look at it, checked the BIAS and it was still OK. I then played the amp some at home at low volumes and again I encountered the same problem. This time it sounded like "knocking on the wall" from the amplifier (power tubes no doubt) I looked at the power tubes and 3 was starting its thing again. I hooked up the bias rite to see if I could catch the bias and voltage but murphys law came into effect and it didn't do it again.... After thought , I took this as a valve fault and returned the valves. 90 day exchange policy.... no biggie.

On the weekend I put in my "bench set" of EL 34 valves A match quad of JJ EL34 tubes. Biased again no worries same as before.

I had the day off today and left early to go home and take advantage of "cranking it" I got into 8 minutes of play and I was on 10 and the wall had a RED FLASH and then the volume dropped immediately.

I removed the chassis from the box and drained the caps labeled everything inside (wires off the main board) and pulled the board.

Fuse FS6 BLEW This is the 1 A 250V fuse for HT power tubes/HT transformer circuit.

I then proceeded to look at the screen resistors (1K ohm 5 watt) They all measure 1.011 K ohms (all 4)

What should I be looking at here.... I am lost.... when the amp "takes off" it seems to be on valves 3-4 nearest the PT. It's always that pair in question.

Could this be a bad choke? I know it regulates B+ but if that was the case you would figure ALL the power tubes would be doing the same thing.....

I successfully bought and repaired many amps but I can't wrap my head around this one.

Sorry for the post but some opinions would be greatly appreciated!!!!

Dan

 

[Jaymz E]

Check your power tube sockets, one might not be getting good contact with the pins.

 

[dan_tramble]

Thanks for the reply I checked the power tube sockets and they all appear OK and are getting contact.

Dan

 

[Jaymz E]

I'm no tech, but could it possibly be that the choke is defective? Check with forum members ampmadscientist or Lane Sparber. Good Luck!

 

[SirArcsAlot]

I have a JVM 410H as well. There are two big (2, maybe 3 watt resistors?) near the bias trimmers inside. These are very out of spec, and mine were blown. They're the 100k resistors to the left and right of the test point. Check them with a multimeter for continuity and then visually, ESPECIALLY THE UNDERSIDE OF THEM.

These could be your problem, as my amp would make a "chirping" noise only when I hit certain notes at extremely high volumes.


I'm not sure what they were replaced as they were replaced by the shop which I am now coincidentally interning at, If I get the chance to open mine up I will.

 

[dan_tramble]

Hi

Do You mean R26/27?

Those are 1 ohm bias supply resistors I presume. Mine test at 1 ohm each

R33/36/39/42 all are a smooth 5.6 k Ohms.

I flipped the board and went over all connections with a fiberglass solder spot electronics brush I may have re flowed 12 spots, was very minimal

Thanks for your opinions appreciated!

Dan

Let me know on the resistor numbers

 

[SirArcsAlot]

Yep 1 ohm my bad. I've had a touch of food poisoning and my eyes are rather blurry. I'll try opening up mine tomorrow.

As for why your tubes are going off like that, maybe try using a chopstick to tap them to see if the socket is bad?

And FWIW I doubt it's the choke causing problems. If the choke were bad it isn't likely your amp would work at all.

 

[mickeydg5]

A problematic choke would affect all power tubes and the preamp.

You should monitor the negative bias voltage when playing like that. A bad component or solder joint in the bias or control grid circuits could be dropping the bias voltage to those power tubes.

It would be beneficial to measure all power tube socket lugs for proper voltages and record.

Side note: power tubes of different brands does not necessarily indicate mismatched tube conditions.

 

[dan_tramble]

Mickey

Yes since I saw only one side go bad, I presumed the choke was fine too.

I am not wondering (I put this question up at JVM forum too) maybe it could be a faulty valve socket? 3

I have to get 1 amp 250 slo blo fuses today to start the amp back up.

So if that is the case, run a valve in 1/4, bias, set to 8 ohms load and play to see what happens?

Thank You

 

[big dooley]

when you have the fuses, put one in and turn on the amp, without any tubes in it...
if the fuse still blows, you have a circuit problem

 

[dan_tramble]

So Dooley

Pull all power tubes and insert the HT fuse 6 and turn on both power and standby switches?

Can I leave the preamp tubes in or pull them as well?

Thank you for your input

Regards

Dan

 

[big dooley]

the problem is propably poweramp related, but since it won't hurt, best is to pull them all and indeed switch on the amplifier in play mode, so that the whole circuit is being active (in stancby mode, some parts are left out of juice)
then one by one insert the tubes and retry
for the powertubes i'd think it would be a smart move to back off the trimpots at minimal (so setting them the coldest) at this point you're troubleshooting and not looking for the best sound being available,
therefore pin 5 on the powertube sockets, should give you the highest negative reading possible

 

[dan_tramble]

Dooley, I beleive you are correct. It is in the poweramp yes.

I got the schematic today and i am wondering something. The valve pair that goes crazy (this happened before) and it happened to pair V1/2 i am wondering here...

let me know if I am onto the correct thought process please...

I look at the OT there are three priamaries. WP3/WP13/WP4

WP3 goes to V1/2 WP 13 goes to the anodes and F6 (1 A that blew) and WP 4 goes to tube pair V3/4

That being said i checked the screens and 1 Kohm, all 4 screens are within tolerance. The two bias resistors are specd' at 1 ohm and the grid resistors are within tolerances too.....

Would it be a safe assumption that WP3 could be "backfeeding" the tube pair V1/2??? I am not wondering now if this is a output transformer problem in disguise...

Would I be able to swap WP3 and WP4 and see if it affected the other side? V3/4??

I found the board diagrams on the metroamp website through google search

Let mer know what you think...

Coincidentaly I got 1 A 250 V time delay fuses today from the electronics shop here, took your advice and fired it up no issues no fuses snapping so far

I'm thinking its the tubes (hard to beleive because a good known set was used in the amp) or the OT

Thanks for your advice/opinions

Regards

Dan

 

[mickeydg5]

Do not swap W3 and W4. Just make sure the connections are solid and the board looks fine there.
Swapping those will cause oscillation and squealing.

I doubt it is caused by the tubes due to the fact you had said different sets had the same result.

If you want to check the OT primary turn the amplifier OFF. Take a resistance measurement from W13 to W3 and then from W13 to W4. What do you have?
You can also measure from W13 to each of the power tube socket lugs themselves at pins #3 (A=anode) to compare readings. They should be about the same on each side of the OT. What do you have?

If it is none of that then refer to post #8 again.

 

[dan_tramble]

Mickey

W3-W13. 15.4 ohms
W4-W13. 13.9 ohms

I pulled 3/4/13 connectors off the board and inserted the test leads into the female connectors to test 3/4/13.

When I tested the OT to the pins I hooked the leads back onto their pins on the board



I test the leads to the pins on the board (3). 0.1 ohms
That's from the OT to the pin socket

What do you think?

Dan

 

[dan_tramble]

UPDATE

Been playing 5 days now, when removed ,main board sucked and reflowed many (all) tube sockets to board and sucked/reflowed the screens as well. The backside (of the board) looks good but the top side there was very little solder on the joints at all.

I just redid them, I wanted to be 110% sure and got good flow on both sides of the solder pad.

Set both sides to 73mV (37 mV per tube) off the bias pins and watched the voltage off the OT and biased to 69% off of 468 V

I set up two test leads to B+ to monitor the voltage (WP3/WP4) and they are both the same and constant so far with no spikes. I presumed that WP3 had an issue seeing that set went red. So far, so good. When the amp was discharged I checked total OT resistance and it was roughly 15 ohms a side (pretty good for a 100 watt)

No red plating at all with the bench set of JJ EL34 in it and the bias stays stable.

Ironically, since I redid the joints, the amp sonically sounds better.. go figure.

Thanks for this input, if situation flips again I will repost

 

[mickeydg5]

Mickey

W3-W13. 15.4 ohms
W4-W13. 13.9 ohms

I pulled 3/4/13 connectors off the board and inserted the test leads into the female connectors to test 3/4/13.

When I tested the OT to the pins I hooked the leads back onto their pins on the board

I test the leads to the pins on the board (3). 0.1 ohms
That's from the OT to the pin socket

What do you think?

Dan

That sounds good.
Hopefully it was just a faulty solder joint.
It is ok to monitor the plate voltages but do not forget to monitor the control grid voltages if need be. Loosing plate voltage will stop the tube from operating but loosing control grid negative voltage will cause excessive operation and high current conditions resulting in red platting.