nielsk
Well-known member
Is that V7 over on the P/S side?
I have a scan on the full manual w/ voltage chart if needed
I have a scan on the full manual w/ voltage chart if needed
RCA BA-6A from scratch
- Thread starter DaveP
- Start date
Help Support GroupDIY Audio Forum:
Hi Nielsk,
It would be very helpful to have a scan of the manual with voltages......post on DIY gmail account?
Otherwise you can mail me through the forum.
The 6H6 is next to the amp tubes and the OD3 is on its own in the power supply section if that helps.
thanks
DaveP
It would be very helpful to have a scan of the manual with voltages......post on DIY gmail account?
Otherwise you can mail me through the forum.
The 6H6 is next to the amp tubes and the OD3 is on its own in the power supply section if that helps.
thanks
DaveP
nielsk
Well-known member
Your email is hidden and I don't know how to post to the DIY Gmail account... are there instructions somewhere?
nielsk
Well-known member
Manual posted in the groupdiy account... ;D
Hi y'all,
I've finished the power supply and the control amp and powered it up. This is what I've found so far:-
1. Had to swap 6SK7's to get a pair that balanced the static voltages.
2. Had to do the same with the 6SJ7's as well. All the voltages came out pretty close to spec.
3. Only had 2 x 6V6 and they were way out. There is no way to balance them in the schematic.
The cathode volts are supposed to be 13.1 which gives just about 40mA/tube with 330 ohms resistors. In fact I got 18.5V on one and 14V on the other. These are NOS 1949 metal tubes.
I put 10 ohm resistors on the plates to measure current and rigged up a single 150 ohm plus 100 ohm pot like a Sta-Level. and then I managed to get 37mA/plate. The trouble with this set-up is that you can no longer use the meter balance circuit for the 6V6's.
4. I could not check the noise because there was so much hum pickup from all these trailing unconnected meter balance wires! I hope they are not going to be a problem.
5. These are the gain figures for the various stages with no GR: 6SK7 gain = 3.52, 6SJ7 gain = 5.38, 6V6 gain = 14.24. 500mV across the 6SK7's gave 135V across the 6V6 plates.
6. maximum output across 600 ohms was 68Vrms at clipping onset(7.7W) this produced a GR voltage on the 6H6 of at least 120V dc.
I've ordered some more 6V6's to try and get a balanced pair. I'll post more pics when these issues are sorted. I'd be interested to hear some input from other BA-6A owners.
best
DaveP
I've finished the power supply and the control amp and powered it up. This is what I've found so far:-
1. Had to swap 6SK7's to get a pair that balanced the static voltages.
2. Had to do the same with the 6SJ7's as well. All the voltages came out pretty close to spec.
3. Only had 2 x 6V6 and they were way out. There is no way to balance them in the schematic.
The cathode volts are supposed to be 13.1 which gives just about 40mA/tube with 330 ohms resistors. In fact I got 18.5V on one and 14V on the other. These are NOS 1949 metal tubes.
I put 10 ohm resistors on the plates to measure current and rigged up a single 150 ohm plus 100 ohm pot like a Sta-Level. and then I managed to get 37mA/plate. The trouble with this set-up is that you can no longer use the meter balance circuit for the 6V6's.
4. I could not check the noise because there was so much hum pickup from all these trailing unconnected meter balance wires! I hope they are not going to be a problem.
5. These are the gain figures for the various stages with no GR: 6SK7 gain = 3.52, 6SJ7 gain = 5.38, 6V6 gain = 14.24. 500mV across the 6SK7's gave 135V across the 6V6 plates.
6. maximum output across 600 ohms was 68Vrms at clipping onset(7.7W) this produced a GR voltage on the 6H6 of at least 120V dc.
I've ordered some more 6V6's to try and get a balanced pair. I'll post more pics when these issues are sorted. I'd be interested to hear some input from other BA-6A owners.
best
DaveP
nielsk
Well-known member
Wow, it is gone.. looks like there is a lot of posts gone. Seems to be an ongoing problem. I'll check into posting it again ???
gemini86
Well-known member
There seems to be someone deleting emails from the Gmail account. If something is missing, please resend it and fwd a copy to groupdiy2 so there's a backup.
gemini86
Well-known member
I'd like to suspect that somebody had added the account to their android pop3 whatever email app and is deleting things not knowing it's deleting them off the gmail account, as well as their phone... BUT, Volker has already disabled that feature, and it seems the account is still being messed with....so now maybe there's just a jerk in our midst...
...or maybe it's the hackers that hit Sony, the CIA and pretty much the rest of the internet.
...or maybe it's the hackers that hit Sony, the CIA and pretty much the rest of the internet.
We are dying to hear how frequency response is working out with the selected transformers.
I have personally never tried to chase vintage power tubes for PP balance, and have always bought modern matched pairs if the type was available. Even then, usually buying x2 or x3 what I need and then selecting in circuit. For these sort of cathode biased power stages, I also usually specify tubes from the lower end of the current measurements. I find many off the shelf modern power tubes draw higher than rated current in vintage cathode biased stages.
Good work!
I have personally never tried to chase vintage power tubes for PP balance, and have always bought modern matched pairs if the type was available. Even then, usually buying x2 or x3 what I need and then selecting in circuit. For these sort of cathode biased power stages, I also usually specify tubes from the lower end of the current measurements. I find many off the shelf modern power tubes draw higher than rated current in vintage cathode biased stages.
Good work!
Well Doug,
I can report this much, I've got 2 more NOS tubes arriving tomorrow so I'll be able to play with the balance then.
I tracked down the hum to the imbalance in the output tubes, its uncancelled hum from the B+.
I rigged up a rough balance circuit and got it down from 42mV to 9mV, so hopefully the new tubes may fix it.
I did a rough frequency response curve (as a straight amp) and its flat to just over 20kHz, but the bass starts to drop below 100Hz though, that may be due to the transformers I guess.
But looking at the manual FR, it looks like the bass switches from dropping from 65Hz to rising from 65Hz under 10dB GR
I did an output Z test on the amp and it worked out at between 110~120 ohms which is in the same ballpark as the Fairchild at 100 ohms.
That's all to report so far.
best
DaveP
I can report this much, I've got 2 more NOS tubes arriving tomorrow so I'll be able to play with the balance then.
I tracked down the hum to the imbalance in the output tubes, its uncancelled hum from the B+.
I rigged up a rough balance circuit and got it down from 42mV to 9mV, so hopefully the new tubes may fix it.
I did a rough frequency response curve (as a straight amp) and its flat to just over 20kHz, but the bass starts to drop below 100Hz though, that may be due to the transformers I guess.
But looking at the manual FR, it looks like the bass switches from dropping from 65Hz to rising from 65Hz under 10dB GR
I did an output Z test on the amp and it worked out at between 110~120 ohms which is in the same ballpark as the Fairchild at 100 ohms.
That's all to report so far.
best
DaveP
The new matched metal NOS 6V6's arrived and they have fixed the imbalance. The noise has gone down to 5mV /70V which is about -83dB below full output. This is as a straight amp with no GR in place.
I tracked the bass loss down to the interstage unfortunately. Still the Gates SA-39 doesn't use one nor do several Collins models. So I disconnected it and subbed in some 470nF caps and HEY! flat from 20Hz to 70kHz now that's what I call AIR. There is a slight dip and then it goes back up to 100kHz before dying for good. So it looks like the Edcor output TX is a good piece of kit.
On the subject of the Edcor, I ordered an 8K:600, which by my reckoning should be 3.65:1 but this one is actually 4.1:1 which works out to 10K:1. Anyway I can now get 70.5VRMS into 600 ohms which is 8.28W, so if you reckon on about 82%TX efficiency there is about 10W through the primary.
I am going to remove the interstage and tidy up the wiring and in my next post I'll include a pic and the revised schematic.
best
DaveP
I tracked the bass loss down to the interstage unfortunately. Still the Gates SA-39 doesn't use one nor do several Collins models. So I disconnected it and subbed in some 470nF caps and HEY! flat from 20Hz to 70kHz now that's what I call AIR. There is a slight dip and then it goes back up to 100kHz before dying for good. So it looks like the Edcor output TX is a good piece of kit.
On the subject of the Edcor, I ordered an 8K:600, which by my reckoning should be 3.65:1 but this one is actually 4.1:1 which works out to 10K:1. Anyway I can now get 70.5VRMS into 600 ohms which is 8.28W, so if you reckon on about 82%TX efficiency there is about 10W through the primary.
I am going to remove the interstage and tidy up the wiring and in my next post I'll include a pic and the revised schematic.
best
DaveP
That's too bad about the interstage. I thought it might be a minor miracle if it were indeed capable of flat bass performance. Henries felt low to me, but I couldn't back that up with technical knowledge. Worth buying the Sowter interstage, I guess.
I'd be curious to know how that interstage performs in a DC coupled single ended plate circuit, say with 5mA through it, if anyone's knocked that up yet.
I'd be curious to know how that interstage performs in a DC coupled single ended plate circuit, say with 5mA through it, if anyone's knocked that up yet.
I've now connected up the GR components and it's right on spec regarding threshold. Verge of limiting is 29.5 db and it max's out at ~30.5 dB. My input TX is only 4:1 so it happens at ~6dB higher input than stock, this is about 100mV.
best
DaveP
best
DaveP
The control amp is now finished and balanced ready for attaching to the front panel.
Most of the trailing wires are for the meter measuring circuits.
I had set the 6SK7's up just on voltages to start with but when I tried the balance controls it cut the residual imbalance down to about 20% of what it was, very impressive.
Power supply needed tweaking, I did away with the resistor between the two 47uF caps to make 100uF, I ended up needing more voltage than anticipated.
Here are the vintage tubes, I am very pleased with them and prefer them to glass now, I really like having the built in screen and the black paint and metal tube conducts the heat away better.
Next week I assemble the front panel.
best
DaveP
Most of the trailing wires are for the meter measuring circuits.
I had set the 6SK7's up just on voltages to start with but when I tried the balance controls it cut the residual imbalance down to about 20% of what it was, very impressive.
Power supply needed tweaking, I did away with the resistor between the two 47uF caps to make 100uF, I ended up needing more voltage than anticipated.
Here are the vintage tubes, I am very pleased with them and prefer them to glass now, I really like having the built in screen and the black paint and metal tube conducts the heat away better.
Next week I assemble the front panel.
best
DaveP
