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VTA ST 120 Rectifier failure


JRS94

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My VTA ST 120 blows the rectifier tube. Last one was 5ar4 tung sol. Happens Every 3 months or so. Usually takes the fuse too. Amp is running off a variac kept at 115v. Power tubes are kt120, Drivers are 5963 hp branded RCA. I keep bias at 450ma via an internal auto bias board. Speakers are khorns. Any ideas?

Thanks

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  • 2 weeks later...

Any luck?

 

My ST-120 has the auto-bias board and a Weber WS-1t (optional thermistor). It's working fine but I had a tough time with the AB install. All fine now. 

The KT-120's bias 0.49, RCA 5963's on the outsides and a Brimar CV4003 in the middle. I'm also on a variac at 117v. Pavel was very helpful as was Holger at Erhard Audio.

Prior to the AB and Weber, I used a TungSol 5AR4 and never had a problem. Amp and variac once blew a fuse but I don't recall why.

 

Have you done the "yellow sheet mod"?  It's on the Dynaco forum.

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I've run KT-120's in my ST-120 amp for several years without blowing a rectifier tube. I'm running a vintage Mullard GZ-33 rectifier tube. I also have some Mullard 5ar4 rectifier tubes that I've had good luck with. I've used the weber copper cap ss rectifiers with good luck also. 

  I have found that the new production Rectifier tubes do not last! I have blown a few new production rectifiers until someone tipped me on buying vintage rectifier tubes. Just my experience.

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This may or may not be helpful, but I ran into a similar issue with an ST-70.  I bought 4 or 5 rectifier tubes and a whole box of fuses.  Eventually, by swapping around power tubes and rectifiers, I was able to find a stable combo.  They have been running steady for several months now without issue.  I don't know exactly what the issue was, but it seemed like some sets of power tubes and/or rectifiers would blow every time, right away.  Others would run for a while and then blow.  With enough experimentation, I was able to find a set that worked.  I guess I'll just go through the same song and dance every time the power tubes need replacement?

 

Amp works and sounds great other than that.

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I had similar problems with an ST-120 I put together as a kit from VTA. Either the rectifier tube would blow, taking the fuse with it, or one of the right side 6550 powers tube would red plate. That got expensive fast. I wouldn't have another kit amp for one very good reason, no warranty. If something goes wrong it's always your fault, it is never a problem with parts, etc. Ask me how I know!

 

I sold that amp, gave the buyer full disclosure in case you're wondering, and eventually bought a 'Chi-Fi' Muzishare X7 (KT88 in push-pull) integrated amp. It has a one year warranty, and as we all know, if anything is going to go wrong, it usually manifests itself in the first few weeks of use. It sounds every bit as good as the ST-120, and it works!

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46 minutes ago, knucklehead said:

I had similar problems with an ST-120 I put together as a kit from VTA. Either the rectifier tube would blow, taking the fuse with it, or one of the right side 6550 powers tube would red plate. That got expensive fast. I wouldn't have another kit amp for one very good reason, no warranty. If something goes wrong it's always your fault, it is never a problem with parts, etc. Ask me how I know!

 

I sold that amp, gave the buyer full disclosure in case you're wondering, and eventually bought a 'Chi-Fi' Muzishare X7 (KT88 in push-pull) integrated amp. It has a one year warranty, and as we all know, if anything is going to go wrong, it usually manifests itself in the first few weeks of use. It sounds every bit as good as the ST-120, and it works!

I've had my ST-120 for 10 years now and still sounding as good as the day I built it. I've had a couple problems with it, but they were all my fault and easily corrected.

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  • 1 month later...

Update, swapped to a Weber WS-1 with thermistor, amp has been running well since then. Bias still moves up a bit for no reason with the AB board, but at least the amp turns on and runs. As stated earlier I'm on a variac, keep voltage to 115v  as much as possible, House voltage swings from 112v to 124v and back quite often. Bias is set at .450v and swings to .600 at which point I flip the amp over and readjust the AB board. Not optimal but not enough to want to change amps. The ST-120 is great with the khorns. 

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11 hours ago, kevinmi said:

I used to think it was the best with my K-Horns until I tried single ended tube amps.

 

41 minutes ago, MechEngVic said:

I'm this close to getting a single ended, just to have both sides of tube-ampery: A PP and a SE.


There is a famous tag line - “Just do it”. nuff’ said. 

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23 hours ago, MechEngVic said:

I'm this close to getting a single ended, just to have both sides of tube-ampery: A PP and a SE.

don't buy a pp amp with switches that switch it to single ended and expect it to sound like a dedicated single ended amp. The Latino amp has this capability, and it doesn't sound anything like my single ended amps.

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2 minutes ago, kevinmi said:

don't buy a pp amp with switches that switch it to single ended and expect it to sound like a dedicated single ended amp. The Latino amp has this capability, and it doesn't sound anything like my single ended amps.

The latino amps have a switch to run the tubes as triodes not single ended. The amp is still operating as a push pull. Having both a st-120 and a tubelab se running 300b, I can confirm they do sound significantly different. 

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2 minutes ago, brl0301 said:

The latino amps have a switch to run the tubes as triodes not single ended. The amp is still operating as a push pull. Having both a st-120 and a tubelab se running 300b, I can confirm they do sound significantly different. 

Whats the difference between a Latino amp in triode and a single ended triode amp? 

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25 minutes ago, kevinmi said:

Whats the difference between a Latino amp in triode and a single ended triode amp? 

A latino amp in triode is running each of the power tubes as triodes but in a push-pull configuration. In a push pull amp, each of the power tubes amplifies etther the positive or negative portion of the signal and then the two halves of the signal are combined again at the output transformer. In a single ended amp, the positive and negative portions of the signal are amplified in the same tube, preventing some of the distortion that occurs in a push-pull amp from when the two havles are combined again.  

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47 minutes ago, kevinmi said:

don't buy a pp amp with switches that switch it to single ended and expect it to sound like a dedicated single ended amp. The Latino amp has this capability, and it doesn't sound anything like my single ended amps.

I'm thinking Decware or one of Tube Fanatics designs.

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