1968 Vibrolux Reverb bias question

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1968 Vibrolux Reverb bias question

Postby jy3 on Sun Jun 19, 2016 3:38 pm

Changing out the power tubes on my amp, and wanted to know if this is accurate to set the bias.

I'm coming up with an idle bias of 24mA.
Using the formula 17.5(half of 35 watts) divided by 460(plate volts) multiplied by 0.65(65% of maximum dissipation).

Am I even in the ballpark? Thanks.
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Re: 1968 Vibrolux Reverb bias question

Postby Baron Von Machinenmann on Tue Jun 21, 2016 1:11 am

6L6GC is 30 watts max plate dissipation.

60% would be 18 watts at idle, 70% would be 21 watts at idle.

18 watts divided by 460 = .039
21 " " = .045

You can go colder as well, use your ears. :wink:
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Re: 1968 Vibrolux Reverb bias question

Postby jy3 on Tue Jun 21, 2016 1:50 am

Baron Von Machinenmann wrote:6L6GC is 30 watts max plate dissipation.

60% would be 18 watts at idle, 70% would be 21 watts at idle.

18 watts divided by 460 = .039
21 " " = .045

You can go colder as well, use your ears. :wink:


Thanks!
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Re: 1968 Vibrolux Reverb bias question

Postby slider313 on Tue Jun 21, 2016 2:37 am

It all depends on the tubes in question. 7581A's are 35 watt tubes, 6L6GC's are 30 watt tubes where 6L6WGB's are a 26 watt tube and 5881's are a 23 watt tube. I shoot for 36ma with the Russian made Tung Sol 6L6GC's in my '64 Vibrolux Reverb. The plate voltage is usually in the 430v range at that setting.
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Re: 1968 Vibrolux Reverb bias question

Postby stratele52 on Tue Jun 21, 2016 7:58 am

On most a Fender amp I fix, I don't hear better sound between, 55/60 to 70 % plate dissipation.
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Re: 1968 Vibrolux Reverb bias question

Postby jy3 on Tue Jun 21, 2016 1:49 pm

slider313 wrote:It all depends on the tubes in question. 7581A's are 35 watt tubes, 6L6GC's are 30 watt tubes where 6L6WGB's are a 26 watt tube and 5881's are a 23 watt tube. I shoot for 36ma with the Russian made Tung Sol 6L6GC's in my '64 Vibrolux Reverb. The plate voltage is usually in the 430v range at that setting.



I'm in that ball park @ around 38mA /450v on the plates. I'm using some Mesa STR-440's which may be Chinese, I'm not sure.

I should probably look for a better quality set, but they're sounding good :D, so I can't complain.
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Re: 1968 Vibrolux Reverb bias question

Postby stratele52 on Tue Jun 21, 2016 7:05 pm

What kind of 6L6 are those STR-440 ?

Answer ? 6L6GC

http://www.allansbillyhyde.com.au/prod/ ... air_in_Box).aspx

For 450 volts bias at 70% are;

46ma for 6L6GC
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Re: 1968 Vibrolux Reverb bias question

Postby pdf64 on Fri Jun 24, 2016 4:18 pm

slider313 wrote:It all depends on the tubes in question. 7581A's are 35 watt tubes, 6L6GC's are 30 watt tubes where 6L6WGB's are a 26 watt tube and 5881's are a 23 watt tube. I shoot for 36ma with the Russian made Tung Sol 6L6GC's in my '64 Vibrolux Reverb. The plate voltage is usually in the 430v range at that setting.

I think that setting an amp's operating point according to the plate limit of the tubes that have been installed is like looking down the wrong end of a telescope, as it ignores the capability of the amp's power supply.
Most Fender BF 2x6L6 amps seem to use similar plate and screen grid arrangements to the 5F6A Tweed Bassman, which was intended for 5881 output tubes, which have a 23 watt plate limit. Therefore it seems reasonable to base the operating point on the rating of the intended tube type; yes 7581A have a 35 watt plate limit, but their key characteristics are otherwise identical to 5881, so why should fitting them mean that the operating point be moved closer to class A? The power supply doesn't become beefier just because tubes with beefier plates have been fitted.
Rather the Vibrolux tends to have a lower rated PT than the Tweed Bassman, which may indicate that it was intended for a cooler operating point.
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Re: 1968 Vibrolux Reverb bias question

Postby jy3 on Fri Jun 24, 2016 9:01 pm

pdf64 wrote:
slider313 wrote:It all depends on the tubes in question. 7581A's are 35 watt tubes, 6L6GC's are 30 watt tubes where 6L6WGB's are a 26 watt tube and 5881's are a 23 watt tube. I shoot for 36ma with the Russian made Tung Sol 6L6GC's in my '64 Vibrolux Reverb. The plate voltage is usually in the 430v range at that setting.

I think that setting an amp's operating point according to the plate limit of the tubes that have been installed is like looking down the wrong end of a telescope, as it ignores the capability of the amp's power supply.
Most Fender BF 2x6L6 amps seem to use similar plate and screen grid arrangements to the 5F6A Tweed Bassman, which was intended for 5881 output tubes, which have a 23 watt plate limit. Therefore it seems reasonable to base the operating point on the rating of the intended tube type; yes 7581A have a 35 watt plate limit, but their key characteristics are otherwise identical to 5881, so why should fitting them mean that the operating point be moved closer to class A? The power supply doesn't become beefier just because tubes with beefier plates have been fitted.
Rather the Vibrolux tends to have a lower rated PT than the Tweed Bassman, which may indicate that it was intended for a cooler operating point.



I thought the earlier Vibrolux's used 6V6GT's up until 1960, then switched to 6L6GC's in '61.

I'm obviously confused. :?
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Re: 1968 Vibrolux Reverb bias question

Postby Baron Von Machinenmann on Fri Jun 24, 2016 11:53 pm

pdf64 wrote:
slider313 wrote:It all depends on the tubes in question. 7581A's are 35 watt tubes, 6L6GC's are 30 watt tubes where 6L6WGB's are a 26 watt tube and 5881's are a 23 watt tube. I shoot for 36ma with the Russian made Tung Sol 6L6GC's in my '64 Vibrolux Reverb. The plate voltage is usually in the 430v range at that setting.

I think that setting an amp's operating point according to the plate limit of the tubes that have been installed is like looking down the wrong end of a telescope, as it ignores the capability of the amp's power supply.
Most Fender BF 2x6L6 amps seem to use similar plate and screen grid arrangements to the 5F6A Tweed Bassman, which was intended for 5881 output tubes, which have a 23 watt plate limit. Therefore it seems reasonable to base the operating point on the rating of the intended tube type; yes 7581A have a 35 watt plate limit, but their key characteristics are otherwise identical to 5881, so why should fitting them mean that the operating point be moved closer to class A? The power supply doesn't become beefier just because tubes with beefier plates have been fitted.
Rather the Vibrolux tends to have a lower rated PT than the Tweed Bassman, which may indicate that it was intended for a cooler operating point.


This isn't a tweed.......It is common practice on many of the tolex era 40 watt Fenders to push the output transformer harder with higher wattage tubes like the 7581A with no ill effects sustained by the power transformer, they have enough extra beef to handle this. As this amp is a '68, the rectifier tube will simply sag when pushed, not an issue IMO.

Want to talk about amp torture? I have a couple of customers that insist on running 6l6GC's in their bf Deluxes.....they want the o.t. pushed to near meltdown for the tones they can coax, these modified (by yours truly) rigs have extra fuses, cooling fans and are carefully biased using tubes I trust, NOS GE grey plates. Haven't lost a patient thus far.
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Re: 1968 Vibrolux Reverb bias question

Postby pdf64 on Sun Jun 26, 2016 4:31 pm

Baron Von Machinenmann wrote:It is common practice on many of the tolex era 40 watt Fenders to push the output transformer harder with higher wattage tubes like the 7581A

I don't see how the fitting of 7581A would push the OT harder, as the 6L6 variants/ equivalents share the same plate characteristics, so given bogey examples of each type, an amp would put out the same max (audio) power?

To re-phrase my hypothesis, most all Fender p-p 6L6 amp 5F6A onwards use the power tube same operating conditions, and I'm not aware of anything to indicate that the intended conduction angle increased with the move from 5881 to 6L6GC.
That's why Fenders with 6L6GC are generally fine with a seemingly cooler bias; they were designed for power tubes with 23 watt plates, and so the same conduction angle / idle plate current can be used whether the plates of the 6L6 variant that happens to be in use are 23, 30 or 35 watts.
If it is desired to increase the conduction angle closer to class A, eg by the use of a reduced control grid bias voltage and the use of beefier 6L6 variant tubes such as 7581A, then yes, it's a great idea to be mindful of the increased stress on the power supply and mitigate the risks with additional fusing (a good idea whatever).
But there's no technical reason why the fitting of a 6L6 variants with beefy plates, eg 7581A, would require that.
That is the point where the 'biasing to 70%of the power tube limiting plate dissipation' method breaks down.
It's helpful as a shortcut for identifying a suitable idle plate current to achieve the intended conduction angle with the tube type that the amp was designed to use.
But not so good if blindly followed when the tube variant is subsequently changed.
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Re: 1968 Vibrolux Reverb bias question

Postby Bigsbyguy on Mon Jun 27, 2016 7:16 pm

I've never re biased my own amp but just got a BF VR. One of the power tubes was giving me odd readings on the tester. The amp works so the plan was to clean the pins well and retest. But I thought about changing them. I've got 6L6's, 7581's and 5881's. I was thinking of using 7581a's.
My tech uses Ken Fisher's bias method. I read up on it in Gerald Weber's book but still am a bit foggy on how to execute it?
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Re: 1968 Vibrolux Reverb bias question

Postby pdf64 on Tue Jun 28, 2016 9:17 am

The current shunt method published by Ken Fisher in the Trainwreck Pages is needlessly risky, and basically inaccurate unless a high spec meter is used. So it may be characterised as all drawbacks and no benefits (apart from being quick and direct / calculation free), most definitely the least suitable method for a person who is not a competent tech to use.
Have you considered getting a bias probe tool?
If you're sure that you are safe to work inside a live amp chassis, there's some discussion and links to further info here http://www.thegearpage.net/board/index.php?threads/concerning-different-biasing-methods.1701437/#post-21889357
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Re: 1968 Vibrolux Reverb bias question

Postby Gasstationman on Tue Jun 28, 2016 9:39 pm

Welcome onboard jy3!!!
Bias set according to strict calculation is overrated.

Baron Von Machinenmann wrote:You can go colder as well, use your ears. :wink:


stratele52 wrote:On most a Fender amp I fix, I don't hear better sound between, 55/60 to 70 % plate dissipation.


Your ears are your best tool. Set bias where it sounds best by ear. I think you end up in the range stratele52 wrote or lower.
Good luck.
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