Looks like this thread has been sent into Exile, aka Instruments and Amps.
Jimmy, are you assuming that the amp uses a 7189A based on the Teisco 10 schematic or is the tube number marked on the chassis? If it's not marked, it's possible that the 5 and 10 refer to watts and that the 5 can, if fact, use a 6BQ5.
If the amp is labelled for a 7189A just as the Teisco 10 schematic I'm wondering if the amps might actually be the same and the "10" in the model number might refer to the size of the speaker used - a 10" rather than a smaller size.
And what about the heater circuit. The schematic must be in error there, right? I think a 6X4 would require a separate heater winding that is center tapped.
Does the heater circuit in the amp match the schematic??
Jimmy, are you assuming that the amp uses a 7189A based on the Teisco 10 schematic or is the tube number marked on the chassis? If it's not marked, it's possible that the 5 and 10 refer to watts and that the 5 can, if fact, use a 6BQ5.
If the amp is labelled for a 7189A just as the Teisco 10 schematic I'm wondering if the amps might actually be the same and the "10" in the model number might refer to the size of the speaker used - a 10" rather than a smaller size.
And what about the heater circuit. The schematic must be in error there, right? I think a 6X4 would require a separate heater winding that is center tapped.
Does the heater circuit in the amp match the schematic??
> I think a 6X4 would require a separate heater winding that is center tapped.
No. 6X4 is not fussy.
No. 6X4 is not fussy.
I think that this is pretty much exactly the same amplifier as the schematic....they might have changed model names of the amps or different areas and markets across the world, as was common with japanese manufacturers in the 60's and 70's. I have tracked back the whole layout of the power tube and it is exactly the same as the schematic, therefore that would lead me to believe it is for a 7198A power tube and that there is no negative feedback as per schematic.
As for the heaters...yes one side is grounded....not too sure why. And btw I have removed the death cap.
As for the heaters...yes one side is grounded....not too sure why. And btw I have removed the death cap.
I think that this is pretty much exactly the same amplifier as the schematic....they might have changed model names of the amps for different areas and markets across the world, as was common with japanese manufacturers in the 60's and 70's. I have tracked back the whole layout of the power tube and it is exactly the same as the schematic, therefore that would lead me to believe it is for a 7198A power tube and that there is no negative feedback as per schematic.
As for the heaters...yes one side is grounded....not too sure why. And btw I have removed the death cap.
As for the heaters...yes one side is grounded....not too sure why. And btw I have removed the death cap.
OK. I think we can work with that. Now here's a hitch. The 7189A has pin 1 & 2 connected internally so the pin solder lugs can be connected like a terminal strip so other components can have a convenient connection point BUT the 6BQ5 does NOT allow this to be done. There is an internal connection to something else inside a 6BQ5 from pins 1, 6, 8. You cannot have anything external going to those pins. A 7189A also has pins 6 and 9 internally connected and that makes those pins also a common solder point for that but if there is an external component going to 6 then it will not work right with a 6BQ5 in the socket.
So the squeeeling could be all about having a 6BQ5/EL84 in there. Once you get the coupling caps changed, I'd say your gonna have to bite the bullet and get a 7189A just to get the amp tested completely, properly. The alternative is to rewire the socket to clear 1 and 6. Then to use an EL84 you'd have to modify the B+ and screen, cathode resistor, and then the load resistors to the drivers. It'll get complicated going that route. But It will probably be a nice little amp with not much wrong with it.
Last edited:
I agree with 20to20, first step is to make sure your output tube is actually a 7189A, not a 6BQ5/EL84 and not a regular 7189. You might still have to tweak a few parts values in order to make sure the voltages are reasonable.
Yes, you could make more changes to the circuit in order to be able to use the more common 6BQ5 but I just took a quick look at auctions for 7189As and found ones that are certainly priced reasonably, along with the more numerous listings with ridiculous prices. While they are not currently manufactured, they are not unobtainable or priced out of reach if you are selective. I would suggest sticking with one of the major US manufacturers (GE, RCA, Sylvania) as I seem to recall finding some Japanese tubes that were labelled 7189A but actually were not.
Yes, you could make more changes to the circuit in order to be able to use the more common 6BQ5 but I just took a quick look at auctions for 7189As and found ones that are certainly priced reasonably, along with the more numerous listings with ridiculous prices. While they are not currently manufactured, they are not unobtainable or priced out of reach if you are selective. I would suggest sticking with one of the major US manufacturers (GE, RCA, Sylvania) as I seem to recall finding some Japanese tubes that were labelled 7189A but actually were not.
The cap you removed is different from the so-called "death cap". The schematic shows a cap across the AC line. That configuration is designed to reduce noise, RFI (radio frequency interference) I think. It should probably be replaced with a modern safety cap rated X1/Y2.
The same type can be used to replace the so-called "death cap", which is installed from the AC line to the chassis ground. It got it's name from the fact that the vintage caps used in that position often leaked voltage or became a short to the chassis if they failed completely. If you touch the chassis of a vintage piece of gear and you feel a little tingle, that is usually leakage from that cap. The modern X1/Y2 rated caps go open if they fail so there's no danger of them dumping AC onto the chassis.
Yep. The 7189A is unique with those pins tied together internally. If you can read any continuity between 1 and 2, and 6 to 9, out of the socket, then you can assume you probably have a 7189A tube if there are no other markings on it.
Actually, I don't think so, especially if you use an EL84M. The most will be a check of dissipation and a rebias if necessary.
Sent from my phone with Tapatalk. Please excuse any typpos.
Well yes I did take the readings between 1 & 2 and 6 & 9 and there are a few ohms between them on the unmarked tube whereas the on JJ EL84 there is no continuity
OK. What's the status on the coupling cap changeout? Also, does that amp have the 1/4" instrument input jack? You may need to hook it up to something to deaden the input or ground it so no stray noise is picked up from the open jack. Then the testing can proceed for voltages with the tube in place. Hope it's a good 7189A. Are you OK in using your meter probes under the chassis for voltage checks?
still waiting for the caps to arrive, hopefully by the end of the week, yes the input jacks are 1/4". Well I have already taken the voltages of all the tube sockets so yes I'm ok with that.
While you're waiting, you could snip out the cap going to the 7189A and see if that has any affect on the squealing. Also, on those prior voltage checks at the socket, it seemed they were done with the tube out of the socket, yes? The cathode voltage on pin 3 wasn't listed and wouldn't be seen without the tube in place.
ok replaced the 0.01uf caps and the 0.02uf cap. Getting low frequency hum. Reverb seems to work. Tremolo doesn't. When turning down the volume to zero the amp gives off a "farting" sound...when turning up the volume to max it starts to squeal.
are you sure about that? I mean after all the discussion about how much filtering a 6X4 should get and now we're going back to the original design?
100%. Put it back to the original design to see how it performs with the new coupling caps. Hum is from insuficient filtering and you reduced it from 40uF to 10uF after the rectifier. The engineers are being second guessed by forum DIYers. See if it works the way it was designed and then you can moddify the livin' %^%&%#@@# out of it, if you feel like it. Good luck, I'm not hanging on to this thread to chase ghosts.
One big reason the 40uF needs to be the first cap is because it is right off the rectifier feeding the B+ to the OPT. It needs to be filtered well. It is not the source of the squealing. Question. When you changed the filter caps did you also do any other circuit changes to follow the model -10 diagram? The diagram shows that the 7189A screen, and the 10uF cap is grounded on the (+) end. That is not right and looks like someone drew in the dot that makes that grounding connection.
If this is a single-ended amp, which I guess it is, then you definitely want s large filter cap there. Otherwise your amp will suffer from 120Hz hum as you turn up the volume (or 100Hz in Europe).I therefore usually prefer to use 100uF in that position if possible, or else use some form of active regulator.
Sent from my phone. Please excuse any typpos.
- Status
- Not open for further replies.