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Post subject: Replacement Valves For 1981 Twin Reverb
Posted: Fri Jan 23, 2015 12:49 pm
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Hi

The 1981 Blackface Twin Reverb was an odd amplifier as it was similar to the silverface amplifier circuits. It had from basic appearance the same valve set as most Twin Reverbs. However, I would like to know which 6L6GC's sound best and have the nicest tones for a Fender Twin Reverb. The other valves for the preamp can also be discussed.

I won't be able to pay for vintage discontinued lines as I aim to get a quad matched set and that alone is very expensive for me. Can anyone help out?


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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Jan 23, 2015 1:37 pm
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You'd be wise to be more concerned with what contemporary 6L6GC can survive the onslaught of nearly 500VDC to the plates. Your amp is of the ultra-linear design and that environment is brutal on power tubes. I'd recommend Tung-Sol's STR series as being the most durable.

Arjay

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Jan 23, 2015 5:31 pm
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I only have one thing to add to this...Damn Glad to See You Post Again Arjay!!!
:mrgreen:
T2

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Last edited by T2Stratman on Fri Jan 23, 2015 6:06 pm, edited 1 time in total.

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Jan 23, 2015 5:56 pm
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^ What he said !! ^
:mrgreen:

Jerry

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Jan 23, 2015 7:19 pm
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Me too! :thumbup:


:mrgreen:


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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Jan 23, 2015 7:45 pm
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T2Stratman wrote:
I only have one thing to add to this...Damn Glad to See You Post Again Arjay!!!
:mrgreen:
T2


I'm up in that too !! :D

cheers!

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Feb 20, 2015 3:03 am
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What about the pre amp set for this amp?

I am getting what I think is a 60 cycle hum although not certain. It sounds like a low frequency power hum. But I am thinking it must be pre amp tubes because the power tubes are a balanced new set.

What are the best replacements for:
3 x 7025
2 x 12AT7
1 x 12AX7A


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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Feb 20, 2015 9:15 am
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Ephestion wrote:
I am getting what I think is a 60 cycle hum although not certain. It sounds like a low frequency power hum.


Time for new electrolytic caps all around, methinks. Filter array, bias supply, and cathode bypass.

Arjay

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Feb 20, 2015 11:09 am
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Well I think the capacitors were changed at some stage and the amp rarely got used in the last 15 years.

1. Would a power tube gone faulty cause the slow blo fuse to go?
2. What are the best tubes to use for the preamp? I noticed hardly anyone makes 7025 Tubes for instance.


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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Fri Feb 20, 2015 4:38 pm
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Ephestion wrote:
Well I think the capacitors were changed at some stage and the amp rarely got used in the last 15 years.

1. Would a power tube gone faulty cause the slow blo fuse to go?
2. What are the best tubes to use for the preamp? I noticed hardly anyone makes 7025 Tubes for instance.


Electrolytics need to be used to keep them "lubed" if you will and so actually not using an amp allows them to dry out. So yeah it is time to get them changed.

Yes a bad power tube can cause a fuse to blow. Make sure you replace it with the same type.

7025's and 12AX7's are the same tube basically. Sovteks 12AX7LPS's are pretty good tubes, they're plentiful, and relatively inexpensive and would be fine for this application.

T2

Edit: Meant to say Sovtek 12AX7LPS's.

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Last edited by T2Stratman on Sun Feb 22, 2015 12:30 pm, edited 2 times in total.

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Sun Feb 22, 2015 8:31 am
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T2Stratman wrote:
Ephestion wrote:
Well I think the capacitors were changed at some stage and the amp rarely got used in the last 15 years.

1. Would a power tube gone faulty cause the slow blo fuse to go?
2. What are the best tubes to use for the preamp? I noticed hardly anyone makes 7025 Tubes for instance.


Electrolytics need to be used to keep them "lubed" if you will and so actually not using an amp allows them to dry out. So yeah it is time to get them changed.

Yes a bad power tube can cause a fuse to blow. Make sure you replace it with the same type.

7025's and 12AX7's are the same tube basically. Sovteks 12AX7WXA's are pretty good tubes, they're plentiful, and relatively inexpensive and would be fine for this application.

T2


+1.
Exactly as T2 wrote, not playing an amp will cause your electrolytic caps to dry out/leak much quicker than regular playing will. Having a charge running through them seems to keep the fluid more stable. In fact in this regard, old Fender tube amps in general are much like motorcycles, the best way to keep them running is to use them often. That way small problems are noticed before they become big problems, and can be addressed forthwith, and not put off/forgotten about for 15-20 years.
I believe the 7025 may have been a proprietary Fender product, and while NOS are available, the 12AX7 will certainly fit, and is nearly identical. There are several manufacturers. As T2 mentioned, the Russian Sovteks are plentiful, fairly cheap, and should last you a long time.
Are you certain you need a complete set of pre-amp tubes? Unlike power tubes, pre-amp tubes don't generally need regular replacement, especially if unused. Have them tested before replacement, unless you have a preferred upgrade.


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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Sun Feb 22, 2015 9:11 am
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nyquilcoma wrote:
I believe the 7025 may have been a proprietary Fender product.


The dual-triode 7025 was developed in the 1950's for military applications and its operating characteristics are similar to the 12AX7 save for an enhanced signal-to-noise ratio.

Arjay

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Mon Feb 23, 2015 10:59 am
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T2Stratman wrote:
I only have one thing to add to this...Damn Glad to See You Post Again Arjay!!!
:mrgreen:
T2


+1,000

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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Wed Feb 25, 2015 8:31 pm
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Retroverbial wrote:
nyquilcoma wrote:
I believe the 7025 may have been a proprietary Fender product.


The dual-triode 7025 was developed in the 1950's for military applications and its operating characteristics are similar to the 12AX7 save for an enhanced signal-to-noise ratio.

Arjay


I did not know that. Thanks for the correction, Arjay. As always, I bow to your superior knowledge on the subject.
Also, I never had any military service, as many of the more expert posters seem to have had. How long after the transistor era had begun did the U.S. armed forces continue to use vacuum tube technology?


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Post subject: Re: Replacement Valves For 1981 Twin Reverb
Posted: Wed Feb 25, 2015 10:06 pm
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I'm going to guess that it must've been sometime in the late '80s that vacuum-tube usage began to fade from the military and that's only because I have many NOS mil-spec tubes in my caddy that date to the mid '80s. While in the service, my shop turned in a lot of tube equipment for salvage around that time and the NSC supply catalog (from which we ordered parts to maintain this gear) still listed a myriad of available tubes. We had ample supplies of them in our shop and those on hand were declared to be "excess" with no recoverability code issued. Thus, when we disposed of the gear, I likewise "disposed" of the tubes required to support it. Enough to fill a GI-issue footlocker.

:mrgreen:

Arjay

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