Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Misc Schematics and Layouts => Topic started by: cometphoton on August 15, 2022, 08:20:41 am
-
Hello, Its my first post and my first Tube amp repair, I really need a schematic. Any support is fine.
I searched this topic "model 62-9026a" and found information that points to this: This might be the same amp as a Gretsch 6162? It features (2) Jensen Special design 10" speakers, (3) 12ax preamp tubes , 1 6973 for the reverb in pre-amp section, (2) 6972 and a 6eu7 in power amp section, and a 5u4 rectifier tube. When I look at the Gretch 6162 schematic its not what I have as it has 2 distinct pre-amps with multiple 12ax7 tube paths. It lacks the 6eu7 too.
My amp has two channels, channel one has two inputs and both work well. Channel two has two inputs and tremelo and reverb circuits. I replaced the electrolytic caps and the wax paper caps. Tested with a dim light bulb circuit, no shorts. Both channels worked fine at first. Later channel one works fine. Channel 2 is not working. I want to trouble shoot channel two.
If anyone has a copy of this schematic then I would be pleased if we can work out something so I can get a copy.
Thanks for the support.
-
Is it in the library?
https://el34world.com/charts/TubeAmpSchematics.htm
-
I just repaired one of these. I will see if I can find the schematic and post it up. The owner gave me one.
This is a great sounding little amp, well worth your effort IMO.
Jerry
-
OOPS, I spoke too soon.
The one I did was a 9028a
can provide a schematic if wanted.
Jerry
-
Is it in the library?
https://el34world.com/charts/TubeAmpSchematics.htm
I can't find one in the Library.
-
Try googling Airline, Wards, National or Valco
You might have to reverse engineer the schematic yourself
-
Try googling Airline, Wards, National or Valco
You might have to reverse engineer the schematic yourself
I searched the WWW for weeks. I do understand the RE process but it would be nice to find it.
Thanks for your comments.
-
A couple ideas:
Did you check the schematic for the Harmony 430? It was made by Valco and sometimes had a 6eu7, sometimes a 12ax7 serving the same function.
And the 6162 sometimes had a 6eu7 as the PI - attached is a schematic.
And - 6eu7 is a 6 volt version of a 12ax7 - the circuit can be adjusted for either.
And - all the 6973 Valcos are pretty similar - if you find the closest schematic is should be pretty easy to use it as a starting point rather than start from scratch. It may be as simple as gluing the preamps from one to the power amp of another. That's pretty much what they did to come up with all the variations.
-
A couple ideas:
Did you check the schematic for the Harmony 430? It was made by Valco and sometimes had a 6eu7, sometimes a 12ax7 serving the same function.
And the 6162 sometimes had a 6eu7 as the PI - attached is a schematic.
And - 6eu7 is a 6 volt version of a 12ax7 - the circuit can be adjusted for either.
And - all the 6973 Valcos are pretty similar - if you find the closest schematic is should be pretty easy to use it as a starting point rather than start from scratch. It may be as simple as gluing the preamps from one to the power amp of another. That's pretty much what they did to come up with all the variations.
Holy Moly Thanks....great help. Thanks again...
-
Hey, Everyone! This form really works, bmccowan is king...he hit the nail on the head! Just what the doctor ordered.
Sound advice about how I should solve my problem with a great approach. Everyone who posted...Thanks for the help its nice to receive help when you ask. Thanks again :worthy1:
-
This Forum has long been a tremendous help for me, so I am more than happy to "pay it forward" when I can. I am no expert on electronics, but I have gained a lot of knowledge about Valco and Gibson tube circuits. Some through research, but most through building, repairing, and rebuilding. Others here provide a wealth of knowledge about all the things I really don't get; like impedance, capacitance, inductance and so on :icon_biggrin:
-
First of all; I have a good electrical background but no experiences with audio circuits as I worked with coordinated motor-drives in industrial applications. We made light repairs on the printed circuit boards. If so it was to replace a output transistor (triac) because we did not have a board in our hands and the process line was down. So I haven't seen a tube circuit since my classes with Mr. Danelie in the 70s so the rust is starting to come off. I did see a few ignitron drives in my days but I figured I would never use that lost knowledge Danelie taugh me ever again. Little did I know what was about to go down here. Anyhow here we go. I dug out my old notebooks and started watching youtube BlueGlow and Mr. Carlson and found you. So you hit the nail on the head with the Valco 430 and Gretch 6162 as they are as close to the Airline Professional 62-9026a as you can get. I didn't do the reverse engineering yet and this is where I am today.
The Valco 430 and the Gretch 6162 are both extremely close to what I have seen in the Airline. Today I have the Airline in semi working condition. Channel 1 sounds terrific and I rate it at 100% but still not out of the woods with Channel 2. When you look at the Valco 430 it really has most of the same inputs and controls features. I think it just has a 12ax7 over a 6eu7 found in the Airline. I used the Gretch 6162 drawings you sent the most.
I had no way to test the tubes so I purchased new ones. Figured I would do a test inside the running circuits by replacing them one at a time. This was an adventure and I was extremely concerned that it would not go well.Let me explain and where did that tube tester go that was in the drug store in town?
So the pre-amp in the Airline has three 12ax7s and one 6973, The power has two 6973's and one 6eu7, and the rectifier is a 5u4. In the pre amp I replaced one 6973, and if you look at the Gretch information you sent me for reference I replaced the V1(trem) and V2(input) with Electro Harmonix 12ax7s, and I used one of the older RCA 12ax7s for V3 Mix Boost. I will explain later why I didn't replace the RCA in the V3 with a new Electro Harmonix too. Also replaced the power section tubes with a matched pair of 6973's and the 6eu7.
Next I replaced all the old paper caps and the electrolytics just in case. Also one resistor 2.2k it was cracked. So its time to power it up.
Test time:
All new 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.
I will keep you posted. Can’t work on the Airline for a few weeks but I will not give up.
-
documents attached for readers
-
Oh yeah - don't give up, you are well on your way. My suggestions are:
You are no longer seeking a schematic but ate trouble shooting, so I'd ask the moderators to move the thread to tweaks and repairs where it'll get more attention.
Good move to replace the electrolytic caps and paper signal caps.
I suggest you clean all tube sockets with De-Oxit and retension the sockets - cracking sound might clear up.
Chopstick the amp - you might find issues.
Turn all controls down, record dc voltage on all tube sockets and post here.
Clean all pots with a lubricating contact cleaner.
After that, you might be good. If not its a deeper, but doable, dive.
-
I already cleaned the amps sockets and pots. I didn't re-solder any connections yet but its on the next list. I will read up on how you refresh tube sockets and get that done too. After that I will take reading on the tubes connections with volume down so we can compare them.
I can inject a Hz type signals into the amp too? Purchased a cheap pc scope with a signal generator. I was planning to start a trace from grid to grid. Not sure if the reverb tank is ok too. It has resistance on both sides so it seems ok. I will try to come up with a hot signal test for that too.
Anyhow back in the old days my response would have been "Rodger Wilco" and Thanks for the support. Give me a week I will be back.
-
Did thy really insert the trem at a fully bypassed cathode?
-
Tested for loose connections with chopstick, re-soldered a few, Refeshed Tube scokets with Pierre Sprey's SILCLEAR Silver Contact Enhancer ( Mapleshade). Then I Replaced the troublesome RCA Tube with new ElectroHarmonix and the reverb circuit is working. Got some cracks from time to time but I shuffled the 12ax7 Electro harmonix tubes till I found a sequence that sounds the best. No more crackle. The old Airline amp sounds pretty darn good now. Thanks to this site. Thanks again everyone for the tips :worthy1:
-
Did thy really insert the trem at a fully bypassed cathode?
Not sure about what you are asking but the Gretch 6162 Schematic notes this.
The Valco H-430 schematic is darn close to the pre-amp features in the Airline I am working on and The Gretch 6162 power amp section is very close to the Airline. Never found an actual Schematic for the Airline but these two helped me find my problems. I gave the amp back to the customer so I can't verify.