Tubes for my 78' Pro Reverb?

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Tubes for my 78' Pro Reverb?

Postby 78ProReverb on Mon Jul 17, 2017 1:37 am

Hey everyone! New owner of a 78 Pro Reverb here. I am in the process of buying new tubes for this thing and had some questions if any one could help? The power tubes are pretty straight forward 6l6's got it... the pre amp side is a slightly confusing to me.. the order goes from left to right as list...... 12AT7...12AX7A.....7025......12AT7......7025......7025. Now my question is can i just slap six 12AX7's in there and be done with it ?

Thanks
78ProReverb
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Re: Tubes for my 78' Pro Reverb?

Postby Dean Jr. on Sat Jul 22, 2017 7:28 pm

First off, has the amp been recapped. If not,then retubing prior to that service is a waste of time, ime.
That said...tubes...
Looking at the back of the amp, the tubes are numbered V1, V2 etc. V1 is the preamp for the Normal channel, V2 is the input preamp for the Vib Ch, V3 is the reverb driver and should be a 12AT7 or 12AU7 imho, V4 is the reverb return and a gain stage for the Vib ch, V5 is the trem function tube, and V6 is the phase inverter....which I prefer to be as Leo di it a 12AT7. Other than V3 and V6, the tube chart calls for 12AX7/7025 tubes. One can sub tubes if one wants. The 5751(gain of 70) and a 12AY7 (gain of 40) are in the same family as the AX. They were designed as tone processors.
Ime, the AT/AU tubes do not do well in other than driver functions. Leo never used them for any other purpose. I just replaced the 12AT7's in the two tone processing preamp stages of a boutique clone of a Princeton REverb with a 5751 in the input stage and a 12AX7 in the reverb return/gain stage position....and the amp came alive with musical harmonic content. The owner of the amp, which is about a month old, was amazed at the distance. IT took two tubes and a good speaker to make a handwired $1800 amp make music. AS it left the factory, the sonics weren't worth $100.
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Re: Tubes for my 78' Pro Reverb?

Postby pdf64 on Wed Jul 26, 2017 8:06 am

78ProReverb wrote:Hey everyone! New owner of a 78 Pro Reverb here. I am in the process of buying new tubes for this thing and had some questions if any one could help? The power tubes are pretty straight forward 6l6's got it... the pre amp side is a slightly confusing to me.. the order goes from left to right as list...... 12AT7...12AX7A.....7025......12AT7......7025......7025. Now my question is can i just slap six 12AX7's in there and be done with it ?

Thanks

Firstly why change all the tubes? Are you certain they are all bad? It's safe to say that modern tubes are not as good as old tubes, it's now an unregulated niche industry. It's of no benefit to remove good old tubes, just identify and replace the bad ones.

Secondly it is beneficial in regard of tone and tube life for power tubes (6L6) to be checked in the amp; they may run too hot or too cold.
If you're not a tech, or haven't got a bias probe and the competence to use it, then it's a job for a tech.

7025 and 12AX7A are a low noise types of 12AX7; nowadays we just buy 12AX7 that have been tested for low noise.
A 12AX7 would have very poor performance and a short life in the V3 reverb driver socket. I suggest that you use 12AT7 in the sockets indicated on the tube chart.
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Re: Tubes for my 78' Pro Reverb?

Postby Dean Jr. on Wed Jul 26, 2017 3:57 pm

^^^^^^+1
I have pulled 45-50 year old U.S. 12AX7/7025 tubes, installed a new production tube...and immediately put the old tube back because the old tube produced better sonics.
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