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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST 120 Problems

    dacer38
    dacer38
    dacer38


    Posts : 10
    Join date : 2015-09-04
    Location : Lakehills Texas
    • Post n°1

    ST 120 Problems Empty ST 120 Problems

    Post by dacer38 Tue Feb 09, 2016 10:25 am

    Turned on my amp the other day and no sound and fuse blown. Replaced the rectifier tube with a JJ GZ34 and the amp now powers with all tubes glowing, however np bias voltage found on any of the power tubes and obviously no sound Sad What next? Also on another note, I live in an area that can have fluctuations and power brownouts. Without breaking the bank, will a good computer UPS be useful to regulate the AC. I believe to be the root cause of the current outage?
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    peterh
    peterh
    peterh


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    Location : gothenburg, sweden
    • Post n°2

    ST 120 Problems Empty Re: ST 120 Problems

    Post by peterh Tue Feb 09, 2016 10:49 am

    dacer38 wrote:Turned on my amp the other day and no sound and fuse blown. Replaced the rectifier tube with a JJ GZ34 and the amp now powers with all tubes glowing, however np bias voltage found on any of the power tubes and obviously no sound :( What next? Also on another note, I live in an area that can have fluctuations and power brownouts. Without breaking the bank, will a good computer UPS be useful to regulate the AC. I believe to be the root cause of the current outage?

    Next task is to measure the voltages internally ( on the 6550 and gz34 as per the dynaco instructons)

    An UPS would be of value as it would prevent the fast fluctuations that is harmful. But a much
    cheaper solution would save your day : a non-resetting fault interrupter, whatever it's called.
    I think skizo has a thread about just such a device here. The idea is that if power fails by
    any reason it will disconnect and needs a manual reset to reestabish power again.

    One thread found :
    https://dynacotubeaudio.forumotion.com/t2732-saved-again-by-the-ground-fault-interruptor


    Last edited by peterh on Tue Feb 09, 2016 10:55 am; edited 1 time in total (Reason for editing : Added link to old thread)
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    arledgsc
    arledgsc
    arledgsc


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    • Post n°3

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    Post by arledgsc Tue Feb 09, 2016 11:37 am

    Check out what the power supply has in common with both L-R channels.  A good place to start is the 4.7K ohm 3 watt resistor soldered to the capacitor can terminals.  Visually inspect the resistor and measure resistance with a meter.  Fully drain the caps before attempting !!!  Inspect components on the preamp/ phase inverter board - especially the electrolytic caps.  

    If the resistor/ caps are OK inspect wiring and install tubes for voltage checks if you feel confident.  Upstream side of the 4.7K ohm resistor will have close to 500V while the preamp/ phase inverter board side close to 400V.

    Also inspect the bias voltage circuit while you have the patient on the operating table.  There is a single diode and some electrolytic caps in that part of the circuit as well.
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    dacer38
    dacer38
    dacer38


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    Location : Lakehills Texas
    • Post n°4

    ST 120 Problems Empty Re: ST 120 Problems

    Post by dacer38 Tue Feb 09, 2016 3:15 pm

    Thanks for the responses guys, I forgot to mention that the original rectifier tube was a 5AR4 which my understanding is the same as the GZ34, however in my paperwork Bob mentions the voltage on the GZ34 will run a little higher, not sure if there is an issue with that but figured it was worth mentioning. I also saw where Bob list what the voltages in various points should be. So sorry I didn't go through that before posting earlier. Unfortunately time ran out and had to shuffle off to work. Hopefully I will get a chance to check the voltages tomorrow. I will post my findings or questions after that. Thanks again.
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    Bob Latino
    Bob Latino
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    • Post n°5

    ST 120 Problems Empty Re: ST 120 Problems

    Post by Bob Latino Tue Feb 09, 2016 10:26 pm

    dacer38 wrote:Turned on my amp the other day and no sound and fuse blown. Replaced the rectifier tube with a JJ GZ34 and the amp now powers with all tubes glowing, however np bias voltage found on any of the power tubes and obviously no sound Sad What next? Also on another note, I live in an area that can have fluctuations and power brownouts. Without breaking the bank, will a good computer UPS be useful to regulate the AC. I believe to be the root cause of the current outage?

    It is possible that the JJ rectifier tube that you just put in the amp is not good. I have seen even NEW tubes come in bad after they have been jostled about by USPS, UPS or FEDEX. Just because the rectifier "lights up" is no guarantee that it is working. With the amp on, measure the DC voltage from pin #8 of the rectifier tube socket to chassis ground. You should get about 475 - 525 VDC on a VTA ST-120. If you get no voltage or a very low voltage, then the rectifier is not good.

    Most of the time a glowing rectifier and a no bias voltage situation at all of the 4 bias measuring points, means that the rectifier is not working.

    Try another rectifier tube.

    About running any tube amp off an AC power conditioner to regulate the AC > Some power conditioners alter the output voltage to the point where the amp is not looking at a true AC sine wave. A tube amp will usually "choke" (blow fuses, rectifier tubes etc.) if it is not fed a true AC sine wave. I recommend running all the VTA amps off a wall outlet. If you run it off a wall outlet, you should always get a true sine wave into the amp. Alternately, you could run the amp off a power strip with a circuit breaker set up so that the breaker will trip on a power surge or any other AC power abnormality.

    Bob
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    dacer38
    dacer38
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    • Post n°6

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    Post by dacer38 Tue Feb 09, 2016 10:51 pm

    Thanks Bob, I was thinking the tube could be bad myself, I did purchase two as an extra, I'll give the other tube a try if the voltages don't check out. Thanks for the explanation about power. I will likely find a surge protector somewhere to use.
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    dacer38
    dacer38
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    • Post n°7

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    Post by dacer38 Thu Feb 11, 2016 3:22 pm

    Yep, turned out to be I received a bad rectifier tube, was a good lesson in the end, I wanted to be able troubleshoot it and see what the voltages looked like before just replacing it. Thanks for the help and power suggestions.
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    wildiowa
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    • Post n°8

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    Post by wildiowa Fri Feb 12, 2016 10:04 am

    Bob...does a Variac maintain the sine wave? Also, what about the conditioners popular with rock and roll combos, rack mount jobbies like the Fuhrman etc?
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    Bob Latino
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    • Post n°9

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    Post by Bob Latino Fri Feb 12, 2016 10:15 am

    wildiowa wrote:Bob...does a Variac maintain the sine wave? Also, what about the conditioners popular with rock and roll combos, rack mount jobbies like the Fuhrman etc?

    Hi - Yes - A variac will maintain a sine wave ... This is the reason I recommend them if your voltage is over 122 VAC for most of the day. NOTE - your line voltage will vary somewhat during the day. In some places in the USA the variance is greater than in other places. Everyone should check their line voltage during off peak times like late at night or early morning. I guess I am lucky where I live because the voltage is fairly steady at 119 to 120.5 VAC virtually all day.

    Bob
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    dacer38
    dacer38
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    • Post n°10

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    Post by dacer38 Sat Feb 20, 2016 11:33 am

    Bob Latino wrote:
    wildiowa wrote:Bob...does a Variac maintain the sine wave? Also, what about the conditioners popular with rock and roll combos, rack mount jobbies like the Fuhrman etc?

    Hi - Yes - A variac will maintain a sine wave ... This is the reason I recommend them if your voltage is over 122 VAC for most of the day. NOTE - your line voltage will vary somewhat during the day. In some places in the USA the variance is greater than in other places. Everyone should check their line voltage during off peak times like late at night or early morning. I guess I am lucky where I live because the voltage is fairly steady at 119 to 120.5 VAC virtually all day.

    Bob


    Looks like I went through another rectifier today, with the volume down I switched over to triode and the GZ34 flashed and went dark. I've also been checking my VAC at various times and seems to be 121-122VAC. So I guess my question is, are these rectifiers really that sensitive? Is my power to high? I ordered a variac as it seems my VAC is a little on the high side, however I haven't hooked it up yet. What would be the optimum voltage for this amp, will also be powering an SP12 as soon as I finish up building it. Any recommendations?
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    Bob Latino
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    • Post n°11

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    Post by Bob Latino Sat Feb 20, 2016 11:44 am

    You could try the Gold Lion or the Tung-Sol 5AR4's? They are a little more expensive but are supposed to be a "premium" 5AR4. Your slightly higher line voltages will shorten tube life .. When your variac arrives., set it for a 118 VAC output and you should get better tube life. NOTE - Do not depend on the meter on the variac for an exact output voltage. Plug your multimeter into the variac's output first and adjust the dial on the variac until your multimeter reads 118 VAC.

    Bob
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    wildiowa
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    • Post n°12

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    Post by wildiowa Sat Feb 20, 2016 11:48 am

    My home voltage runs between 124v and 126v. After many issues I finally got a Variac and set it to 119v. As I understand it though the final output will depend on the input...if it's 124 it's going to be lower than if the line is 126. I don't have time to hassle with constantly monitoring and changing it, however, and figure a little low is better than very high. It's probably running somewhere between 117 and 120, which is within the original specs for most old tube stuff I believe. I just think 125 or 126 is a bit much.

    Also just wondering....is it OK to switch modes with your amp running, even with the volume down? Would that cause a spike or something that caused the fault? If that was the "event" just wondered if that was something to avoid.
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    Bob Latino
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    • Post n°13

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    Post by Bob Latino Sat Feb 20, 2016 12:16 pm

    wildiowa wrote:

    Also just wondering....is it OK to switch modes with your amp running, even with the volume down? Would that cause a spike or something that caused the fault? If that was the "event" just wondered if that was something to avoid.

    On any of the VTA amps this should not cause a problem. The triode/pentode ultralinear switches are "make before break" switches. You do get a little "blip" sound when you make the switch but this will not hurt the amp. I have switched my amps here hundreds of times with no ill effect other than a small noise.

    You do make a point though .. Until dacer38 posted about the loss of a rectifier tube upon the switch from triode to pentode ultralinear or vice versa, no one has ever reported to me (since 2007) the loss of tube upon switching the amp's mode of operation.

    Bob
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