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PostPosted: Sat, Jul 06, 2019 2:36pm 
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Hack

Joined: Fri, Aug 05, 2016 11:31am
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Help and advice is appreciated.

I’ve owned my VH4 for 5 years (new 2014). I’ve played it pretty regularly. I decided to change the stock tubes. I replaced all with Ruby tubes. V1 and V2 12Ax7AC5 HG+, the rest 12Ax7AC5 HG. Replaced Power tubes with a matched quad EL34BSTR. I checked the bias of the stock KT77s using the shunt method before removing the tubes. Yel/red had 70mA and Blu/red had 77mA.

When I replaced all the tubes, turned on the amp to let it warmup for 5 min. or more. The amp was making a weird noise. I connected the leads to Blu/red and the reading was at 90-100mA but fluctuating. I quickly turned the bias down to 65mA, but it slowly climbed back up, then it would drop back down. Then the fuse popped for tube 4.

I’m in the process of trying to find another fuse locally, but that is proving a hard find. I also am wondering if this is a direct indication that this tube is bad. I want to contact the supplier, but I want to be sure that there is no more testing that I need to do.

I am an “electrician” at my work, so I respect electricity, but mostly deal with 120Vac - 13.8kV. I’m not an electronics tech. I do know that a blown fuse usually is not a random thing and is caused by a fault condition. Do I need to look farther than the tube?


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PostPosted: Sun, Jul 07, 2019 7:57am 
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Diezel Amplification
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Hello Zinnmaster,

I had a lot of bad EL34BSTR were brand new. Therfor we use
the KT77-JJ since one decade.

You can get the European style fuse (5x20mm) on any good
electronic store. For the VH4 it´s 0,25 amps or 250 milliamps amps slow blow.

Please check also the 4 screen resistors on top of the sockets.
These are white 5 watt resistors and should have about 1 kOhm.

Btw, it´s not ch4 it´s tube fault 4 ;-)

Best regards,
Peter


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PostPosted: Sun, Jul 07, 2019 5:09pm 
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Hack

Joined: Fri, Aug 05, 2016 11:31am
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Thank you Peter for the response, help, and correction! 😁

All of the resistors are around .977 kOhm, so yes 1kOhm. 👍

I will be calling around and asking different stores if they have the fuses. I like that you use ceramic, but are a little more hard to find. I had already found on Amazon, but they will take two weeks to arrive.

I like the KT77-JJ, but I wanted to try the EL34. I was hoping for a little more growl. Sometimes ch 3 and ch 4 seemed a little dark or muddy. Would lowering the bias on the KT77 help? Maybe just the tubes are 5 yrs old? Anyway, I was just trying something different.

Thank you for the help!


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PostPosted: Sun, Jul 07, 2019 6:59pm 
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Plank Cranker Trainee
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Yeah, it's not unheard of to have a brand new quad of power tubes show up with one tube DOA or wildly out of bias range vs the others. Your tube vendor should look after this with a minimum of hassle. If not, find another tube vendor.
Having said that, I have found that KT77's are the best sounding tube in my VH4, and I've tried just about everything including 6550's.
My solution to open the tone up a bit and get a bit more aggression on ch 3 & 4 was to move the negative feedback wire on the speaker jacks "down" one level to get a bit less NFB. I cant remember if it was on the 16 ohm stock, and I moved it to the 8, or if it was on the 8 and I moved it to the 4 ohm, but you get the idea. Easy to do and completely non-invasive ( no messing with the PCB etc.)
Maybe I'm just old and deaf, but that little fix absolutely did the trick for what I was looking for out of ch 3 & 4.
I love my VH4 :inlove:
Incredible sounding amps :rock:

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