Inadvertently left my St120 on for 2 days straight with out music playing. Turn the volume up this morning and no sound. Got the meter to checked BIAS and zero BIAS to all 4 tubes. All tubs are glowing normal. What's the next step? All wires are plugged in. Preamp is emitting power and sound to other amp. Could it be the rectifier tube went south? Stock amp except upgraded capacitors.
4 posters
Zero Bias all 4 tubes and no output to speakers
eminence1963- Posts : 21
Join date : 2019-06-10
Age : 60
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
eminence1963 wrote:Inadvertently left my St120 on for 2 days straight with out music playing. Turn the volume up this morning and no sound. Got the meter to checked BIAS and zero BIAS to all 4 tubes. All tubs are glowing normal. What's the next step? All wires are plugged in. Preamp is emitting power and sound to other amp. Could it be the rectifier tube went south? Stock amp except upgraded capacitors.
The rectifier probably did "go south". Just because a rectifier "lights up" is no indication that the rectifier is working properly. A no bias situation on ALL output tubes is usually a sign that the rectifier is bad.
Bob
ttocs and Gabriel like this post
eminence1963- Posts : 21
Join date : 2019-06-10
Age : 60
- Post n°3
Re: Zero Bias all 4 tubes and no output to speakers
by eminence1963 Fri Oct 22, 2021 6:31 pm
Will this work as a temp sub un-till I can order a new tube
https://www.tubedepot.com/products/solid-state-rectifier
https://www.tubedepot.com/products/solid-state-rectifier
Last edited by eminence1963 on Fri Oct 22, 2021 6:35 pm; edited 1 time in total (Reason for editing : correction)
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
eminence1963 wrote:Will this work as a temp sub un-till I can order a new tube
https://www.tubedepot.com/products/solid-state-rectifier
Yes - That solid state rectifier will work BUT ..
1. That rectifier does not have slow warm up, it will instantly turn the high voltage on before the the output tubes have had a chance to warm up. Output tubes don't like to be hit with 490 - 525 volts DC (as found on the VTA ST-120) with the tube filaments not fully lit. If you want to try that solid state rectifier, I recommend that you install the VTA TDR (Time Delay Relay). The TDR will allow the filaments of the tubes to warm up for about 17 to 20 seconds before high voltage is applied to the output tubes.
2. The solid state rectifier is not a good idea if your line voltage is over about 121 VAC. A solid state rectifier will cause the DC high voltages to be slightly higher than a tube rectifier. The quad cap on ST-120's is rated at 550 volts. If you exceed that voltage in your amp, you can take out the quad cap.
Bob
Dave_in_Va- Posts : 443
Join date : 2013-04-02
Location : Mid. VA
- Post n°5
Re: Zero Bias all 4 tubes and no output to speakers
by Dave_in_Va Sat Oct 23, 2021 12:14 am
A local music/guitar store should have a 5AR4/GZ34 in stock.
I'd play it safe.
I'd play it safe.
eminence1963- Posts : 21
Join date : 2019-06-10
Age : 60
- Post n°6
Re: Zero Bias all 4 tubes and no output to speakers
by eminence1963 Sat Oct 23, 2021 2:54 pm
Ok will wait for the tube to come in.Bob Latino wrote:eminence1963 wrote:Will this work as a temp sub un-till I can order a new tube
https://www.tubedepot.com/products/solid-state-rectifier
Yes - That solid state rectifier will work BUT ..
1. That rectifier does not have slow warm up, it will instantly turn the high voltage on before the the output tubes have had a chance to warm up. Output tubes don't like to be hit with 490 - 525 volts DC (as found on the VTA ST-120) with the tube filaments not fully lit. If you want to try that solid state rectifier, I recommend that you install the VTA TDR (Time Delay Relay). The TDR will allow the filaments of the tubes to warm up for about 17 to 20 seconds before high voltage is applied to the output tubes.
2. The solid state rectifier is not a good idea if your line voltage is over about 121 VAC. A solid state rectifier will cause the DC high voltages to be slightly higher than a tube rectifier. The quad cap on ST-120's is rated at 550 volts. If you exceed that voltage in your amp, you can take out the quad cap.
Bob
Gabriel- Posts : 38
Join date : 2022-01-23
I experienced a similar issue. Replaced the 5ar4 rectifier.
I also used a W3ber solid state rectifier with the delay board, until the rectifier almost melted …slight exaggeration lol and the power transformer was hot enough to cook an egg.
I will nevergo back to a solid state rectifier.
The 5ar4 is perfect..
I also used a W3ber solid state rectifier with the delay board, until the rectifier almost melted …slight exaggeration lol and the power transformer was hot enough to cook an egg.
I will nevergo back to a solid state rectifier.
The 5ar4 is perfect..
Dale Stevens likes this post
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