Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on August 08, 2020, 12:39:56 pm
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I made a p2p conversion of the Custom Vibrolux reverb and noticed that the factory set bias (not adjustable) was only 25mA/tube on 418V plate voltage.
There is certainly a reason for that low value. Do you think there is capacity in the PT for hotter tubes? The amp is manufactured in mid 90's.
BTW. The hiss is gone after the conversion.
/Leevi
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There is certainly a reason for that low value. Do you think there is capacity in the PT for hotter tubes?
Most likely. I would replace R59 with a 22K pot and 10K resistor (connected in series) and experiment. Be aware that adjusting the bias pot will also affect the tremolo strength.
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I have changed the bias adjustable with a 10K pot + 15K resistor and it works fine.
Tremolo is not an issue since I use your Tremor-Lator in the preamp.
I think the current can be increased from 25mA upwards but the 50mA level (70%)
is probably not recommended?
/Leevi
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I bias amps at 60 %
70% is too hot if you crank the amp.
Listen tone at 60 % and 70% you won't hear any difference ( and same at 50 % ? ).
60% will save tubes
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... 70% is too hot if you crank the amp.
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That varies with the amp design, depending on supply voltage & OT loading.
IMO, it's just as incorrect to say "70% is too hot" as it is to say "70% is the right idle bias."
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... 70% is too hot if you crank the amp.
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That varies with the amp design, depending on supply voltage & OT loading.
IMO, it's just as incorrect to say "70% is too hot" as it is to say "70% is the right idle bias."
It is from my experience with Fender amps, the subject .OP is free to bias like he want.
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In this case there is not any idea to bias the amp to 70% level. Deviation grom the Fender factory setting would be too high. Currently it is set to 35%.
/Leevi
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35 % is a way too cold. You need to replace some resistor to have less negative voltage
Best is to put a 20 K pot with 10 K in series to replace 18 K
Custom Vibrolux bias circuit
18K ; put smaller value
Or 2.7 k bigger value
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... OP is free to bias like he want.
Of course.
Just consider that:
- Everything from 0% idle (Class B) to 100% idle (Class A) is the range of operation we call "Class AB."
- The key difference between Class A & Class B is how large Peak Plate Current rises, as a % of idle current.
- Class A can't (cleanly) exceed 2x idle current.
- Class B is many-many times (non-zero) idle current
- The point of Class B (and the cooler end of the Class AB spectrum) is to cool the plate because the high peak currents tend to overheat it.
- So amps producing much more output power (higher peak current) need to cool longer; they should idle cooler than lower-power (Class AB) amps.
At another forum, someone asked about idling the 6L6GCs in a MusicMan amp to 40%. This was rejected (and would be counter to MusicMan's instructions), as the tubes run in Class B2 to deliver 65w from a pair of 6L6GC. They have to be cool-idling to avoid overheating when things get loud.
As for the subject Custom Vibrolux Reverb, the original 25mA (35%) was probably smaller than necessary, but that idle bias would have varied with the tubes used (they don't all land on the same plate current when fed the same bias voltage). Here Leevi could idle hotter. He could scope the amp for max clean output power & bias the amp where most-power is achieved (if max clean output power is a goal). Or he could bias hotter for earlier output tube distortion, as long as no redplating is observed. And there are other ways to solve the problem.
In this case there is not any idea to bias the amp to 70% level. Deviation grom the Fender factory setting would be too high. Currently it is set to 35%.
That's not necessarily the "factory setting."
I have a tube tester that check idle current, as well as transconductance (and other measurements). For one test condition, "ideal plate current" is considered to be 72mA, with an acceptable range of 58-86mA (see Page 2 of this 6L6GC data sheet (https://frank.pocnet.net/sheets/093/6/6L6GC.pdf), left-column of "Class A Amplifier, Pentode Connection"). This tester allows setting of plate, screen & bias voltages to exactly replicate the data sheet condition.
When I tested ~30 6L6GCs, they fell in a range from 60-88mA.
But I had a pair of American made 6L6WGBs that I consider "defective." In the same test with the same fixed bias, these tubes idled at 132-140mA! I tried running them in an amp, and had to set the bias supply for its maximum-negative voltage just to keep these tubes in the "way too hot" range.
Point? When you see tubes with stickers for matching showing Plate Current as well as Transconductance, it's because the idle current for a tube when voltages are applied varies among tube samples. It could be that your present tubes simply have low plate current for the applied voltages. Tell me your actual Pin 3,4, 5 voltage, and I'll tell you how that 25mA compares to other 6L6s.
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Pin 3 = 418 volts at 35 % power dissipation
At 60 %, plate voltage will drop a lot.
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Unfortunately the amp is already on the way to customer. Custom Vibrolux reverb does not have adjustable bias. Before I started the p2p conversion I measured the bias which was 25mA on 418v plate. The bias circuit output voltage was -45v which is also stated in the schematic. I got the same numbers in the converted p2p circuit when setting the voltage to -45v. My interpretion is that the default value is that 25mA which is reached by setting the voltage to -45v. Furthermore I found a discussion from another forum regarding 68 Vibrolux reverb where the non-adjustable bias was 24mA.
https://www.thegearpage.net/board/index.php?threads/fender-service-manual-says-to-bias-6l6s-at-24-ma-68-cvr.1960160/ (https://www.thegearpage.net/board/index.php?threads/fender-service-manual-says-to-bias-6l6s-at-24-ma-68-cvr.1960160/)
The scale of the bias control is not a problem.
The question is rather how hot it is safe to be biased without breaking the PT.
/Leevi
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Short Answer: the pair of 6L6s you were using idle cooler-than-normal. You could have upped the current to 30mA per tube. In fact you could have gone a whole lot warmer, since you'd removed the bias-vary trem circuit.
... 25mA on 418v plate. The bias circuit output voltage was -45v which is also stated in the schematic. ...
My mistake! I should have seen it off to the side.
Measuring an average pair of 6L6GCs, I get 30mA and 31mA when -45v bias is applied in my 1967 Super Reverb. Plate voltage is 414vdc.
... discussion from another forum regarding 68 Vibrolux reverb where the non-adjustable bias was 24mA. ...
The "Custom Vibrolux Reverb (https://el34world.com/charts/Schematics/files/Fender/Fender_custom_vibrolux_manual.pdf)" and "68 Custom Vibrolux Reverb (https://markweinguitarlessons.com/forums/attachments/68-custom-vibrolux-reverb-schematic-pdf.37894/)" are 2 different amps, though they both have bias-vary trem.
The latter shows an onboard bias test point ("TP33") indicating 60mA for a pair of output tubes, or ~30mA per tube.
So the tubes you were using run cooler than normal (either from Fender's schematic or my test with a pair of tubes that land on data sheet specs). You could have biased warmer (30mA and 418vdc yields 12.5 watts, or ~42%).
... I think the current can be increased from 25mA upwards but the 50mA level (70%) is probably not recommended?
... At 60 %, plate voltage will drop a lot.
I turned up my Super Reverb from 30mA to 50mA per tube. Plate voltage came down to 402vdc, and the required bias voltage was -36.4vdc. That's 20.1w and 67%.
The smaller bias voltage means the tubes will distort with a smaller drive signal (~36v peak rather than 45v peak). Gm will be higher, so power output will be roughly the same.
It would take more investigation with a scope (and maybe an IR thermometer) to know if clean power output rises/falls, and if the tube is getting too hot at max output power.
... Tremolo is not an issue since I use your Tremor-Lator in the preamp. ...
I think you already know that as Sluckey said, the cool bias is to make sure 6L6 get enough trem-depth when using bias-vary trem. But you removed that requirement by using a different circuit in the preamp. So you could have biased very much warmer.
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The question is rather how hot it is safe to be biased without breaking the PT.
/Leevi
The fuse will blow before.
The Power tubes will blow before PT
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Short Answer: the pair of 6L6s you were using idle cooler-than-normal. You could have upped the current to 30mA per tube. In fact you could have gone a whole lot warmer, since you'd removed the bias-vary trem circuit.
... 25mA on 418v plate. The bias circuit output voltage was -45v which is also stated in the schematic. ...
My mistake! I should have seen it off to the side.
Measuring an average pair of 6L6GCs, I get 30mA and 31mA when -45v bias is applied in my 1967 Super Reverb. Plate voltage is 414vdc.
... discussion from another forum regarding 68 Vibrolux reverb where the non-adjustable bias was 24mA. ...
The "Custom Vibrolux Reverb (https://el34world.com/charts/Schematics/files/Fender/Fender_custom_vibrolux_manual.pdf)" and "68 Custom Vibrolux Reverb (https://markweinguitarlessons.com/forums/attachments/68-custom-vibrolux-reverb-schematic-pdf.37894/)" are 2 different amps, though they both have bias-vary trem.
The latter shows an onboard bias test point ("TP33") indicating 60mA for a pair of output tubes, or ~30mA per tube.
So the tubes you were using run cooler than normal (either from Fender's schematic or my test with a pair of tubes that land on data sheet specs). You could have biased warmer (30mA and 418vdc yields 12.5 watts, or ~42%).
... I think the current can be increased from 25mA upwards but the 50mA level (70%) is probably not recommended?
... At 60 %, plate voltage will drop a lot.
I turned up my Super Reverb from 30mA to 50mA per tube. Plate voltage came down to 402vdc, and the required bias voltage was -36.4vdc. That's 20.1w and 67%.
The smaller bias voltage means the tubes will distort with a smaller drive signal (~36v peak rather than 45v peak). Gm will be higher, so power output will be roughly the same.
It would take more investigation with a scope (and maybe an IR thermometer) to know if clean power output rises/falls, and if the tube is getting too hot at max output power.
... Tremolo is not an issue since I use your Tremor-Lator in the preamp. ...
I think you already know that as Sluckey said, the cool bias is to make sure 6L6 get enough trem-depth when using bias-vary trem. But you removed that requirement by using a different circuit in the preamp. So you could have biased very much warmer.
Very good summay
Thanks
/Leevi
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... BTW. The hiss is gone after the conversion. ...
Did you start with the blackface Custom Vibrolux Reverb, or the silverface 68 Custom Vibrolux Reverb?
Did you have to get rid of the "reverb on both channels" feature? Or add negative feedback?
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It was blackface "Custom Vibrolux Reverb" from mid 90's. Channels are combined in the 3rd tube i.e. the reverb is working in both channels.
https://el34world.com/charts/Schematics/files/Fender/Fender_custom_vibrolux_manual.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_custom_vibrolux_manual.pdf)
I kept the preamp structure as it was except the the trermolo which is placed before rhe PI.
At least the following changes havebeen done (see the pics):
- Tremolo moved to preamp (Tremor-Lator)
- Pre-PI master added (back panel)
- NFB with pot (back panel)
- Adjustable bias with measurement points for multimeter (1R resistors)
- Surge protection diodes removed
- Power tubes grid stoppers added
- Tone stacks done according to 68 Vibrolux reverb (because of lack of special treble pots)
- A bigger coupling cap (2.2n instead of 500p) changed to reverb recovery
- A bigger bright cap (100p instead of 47p) changed to bright channel
- Reverb tank covered with cardboard
The result is very silent amp i.e. no hum or hiss.
/Leevi
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That’s a lot of shielded cable in that amp.
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That’s a lot of shielded cable in that amp.
Yes, I normally don't use that much. On the input and tremolo it is a must in order to prevent ticking even if tremor-lator is used. Otherwise there is lot of amplification and the distances are relative long so I thought to do it "ready" right away.
It's probably overkill but I don't care😊
And I'm aware of the capacitance of the shieded cables.
/Leevi
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". I turned up my Super Reverb from 30mA to 50mA per tube. Plate voltage came down to 402vdc, and the required bias voltage was -36.4vdc. That's 20.1w and 67%.
-Leevi
It is better, test tone at 60%.
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The question is rather how hot it is safe to be biased without breaking the PT.
/Leevi
The fuse will blow before.
The Power tubes will blow before PT
Regarding fuses, they only blow quickly when there's a significant fault condition; continuous overcurrent can be sustained for hours.
The latter statement seems speculative and runs counter to my experience.
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It is my experiences.
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It is my experiences.
Hence the benefit of sharing knowledge and experience, eg via forums such as this. As no one knows everything, and even super brains will make mistakes / get confused once in a while.
I'm not claiming that transformer damage is a common or typical outcome of a power tube failure, rather that it's possible and does occasionally happen.
Fuses will sustain prolonged but mild overcurrent (ie without blowing), whereas the insulation of a transformer winding may not.
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> I'm not claiming that transformer damage is a common or typical outcome of a power tube failure, rather that it's possible
It "can". We had an Ampeg VT-40. One of the 7027 power bottles shorted Screen-Cathode (only when hot, hard to diagnose). The power transformer burned beyond easy redemption. Back around 1990 when tube-parts were hard to find. (Around 2005 I stuck a web-ordered Fender-clone PT in to get it going again.)
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And screen to cathode shorts are exactly the fault mode where a 1 watt screen grid resistor will help to mitigate, and a beefy 3 or 5 watt resistor can only assist, in the transformer destruction process.