Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: cometphoton on August 20, 2022, 07:51:42 am
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I have a question, Why do the RCA 12ax7 tubes have more gain than the Electro Harmonix. So when I install the Electro Harmonix my amp has a termendous loss of power ?
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… when I install the Electro Harmonix my amp has a termendous loss of power ?
I take that sort of thing as a sign that the valve may be bad.
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When i buy tubes from any supplier the first thing i do is test them. I have learned over the years to not assume anything just because it is new and in a box. I find quiet a number of weak new tubes and the filament warm up time in excess of what i consider to be normal. In comparison to vintage tubes there is little other than appearance. The metal inside the tube is cheap tinsel tin foil in comparison to a vintage RCA,Sylvania etc. There is very little nickle in the metal used today to give a component structural strength. It all comes down to the manufacturing process and quality control. Every batch can be different than than last batch. JMHO
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+1 for bad tube. The EH 12ax7 from the past decade is consistent and above average for modern production.
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This is what I found, The Electro Harmonix tube are all aok. My amp has a strange operation in one circuit when I change from RCA to Electro Harmonix. I only found this by trial and error replacing tubes one at a time. Let me explain
The Amp in question is a Wards / Airline professional 2 channel that has Tremelo/Reverb on channel two, model 62-9026a. Can't find a schematic for it but on this form I found help and its almost like a combination of the Gretch 6162 and the Valco 430. The airline has a pre with (3) 12ax7's and one 6972 and a power section using twin 6973's and one 6eu7. The rectifier is a 5u4.
So
All new EH tubes: Channel 1 worked fine. Channel 2 had mixed problems. I found it making loud cracking noises with no input and volume down. At some point it would work at low volume but it always ended up by me turning the amp off. Once the cracking started it would not stop unless you powered down.
Finally had channel 2 up and running clean when I used a RCA 12ax7 in V3 (mix boost). So if I used the older RCA 12ax7 in V3 (reference Gretch schematic) as mix boost, Channel 2 was up and working. Note both channels 1 and 2 have two inputs (reference in Valco 430 schematic as Treble and Bass). On the Airline they are marked treble and regular. Little differences in brands I guess.
With trials I changed each tube one on one and found I could use RCA tubes for V3 mix boost, V1 Trem and V2 input or Electrol Harmonix in V1, V2 but only with a RCA in V3. I found two older 6973 good as new Electro Harmonix too. Not sure what is going on here between RCA and Electro Harmonix? But I didn't go down that rabit hole yet.
Now in Channel 2 the Tremolo circuit works fine but I really can't distinguish any changes in the Reverb sound when I turn the foot switch on and off. So I know this circuit requires more work.
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Images of the amp attached
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I have posted to your other thread on this amp in misc. schematics. I suggest you, or moderators combine the threads. I also suggest that you clean and tension all tube sockets. I do not believe that there is enough performance difference across tube brands/vintages to cause the issues you experience. If spiffing up the sockets does not help, you may want to record and post all socket pin dc voltages and chopstick the amp for bad solder joints.