12BH7A but can't test - UK Vintage Radio Repair and Restoration Discussion Forum

Longespee · 1st Apr 2010, 8:04 pm

Perhaps I'm being a little dumb here but, I have a 12BH7A to test on my AVO mk2 and I dial up 741 226 413 (12BH7 in ed 17 AVO Data ) and it works out as REPLACE, then I realize it has an A but my list doesn't cover this one, but Ed 20 does and lists as 841 238 410, so I dial this up and get no readings at all, anyone through some light, cheers, Sean.

Tim · 1st Apr 2010, 8:22 pm

If it's any help the 12BH7 has the same pinout as ECC81,82,83 etc.

julie_m · 1st Apr 2010, 8:23 pm

If you are using a 6V3 heater supply, then you definitely need @@@ 22@ @@3. You would need @@@ 23@ @@0 with a 12V6 heater supply. (Thanks to Top Cap for explaining what the dials do .....)

You could try 641 220 003 for the left-hand triode, and 000 226 413 for the right-hand one? See also the data sheet here.

Longespee · 1st Apr 2010, 8:52 pm

Thanks guys, I will have a go. Could I, as the 12BH7A has a voltage of 12.3, test it as a 6V with the 741 226 413 and get a reading of goodness. Although testing earlier on 741 A1 was under spec but A2 was no reading.

Station X · 1st Apr 2010, 9:35 pm

When you say it says replace is this for A1, A2 or both? It's possible that one half of the heater is open circuit. This will only show up on the tester if the heaters are connected in series across a 12V supply. Unless of course both halves of the heater are O/C, which will show up on a 6V supply with the heaters in parallel.

Whenever I test a new type of valve for the first time I always check the thumb switch settings against the valve data and then tick the entry in the AVO data book.

Station X · 1st Apr 2010, 9:47 pm

The settings 841 238 410 cannot possibly be for a double triode, as no switch is set to 6 or 7 which would be the anodes.

Longespee · 1st Apr 2010, 9:50 pm

It's all a little out of my depth really, but if I test as 741 with 6v settings then A1 is replace and A2 won't read at all, but with Edition 20 and 12BH7A settings at 841.. and 12v it won't read and all is zero.
I need to sit down and go through the heater test in theory to get the whole thing to register brain wise first.

Station X · 1st Apr 2010, 9:58 pm

The difference between the 12BH7 and 12BH7-A is explained here:-

http://www.jogis-roehrenbude.de/Roeh...BH7/12BH7A.pdf

Take it from me the 841 settings are wrong as they relate to testing a diode.

741 226 413 is correct for a 6V heater voltage setting.
741 236 410 is correct for a 12V heater voltage setting.

An easy option would be to test the heaters for continuity with a meter on ohms range. Check betwen pins 9 & 4 and 9 & 5.

Longespee · 2nd Apr 2010, 10:15 am

Many thanks Graham, I have tested at 741 236 410 and I can get a reading on A2, alas the two valves I have test replace.
Another amendment to the AVO book I think.

Station X · 2nd Apr 2010, 10:20 am

Are you saying there's no reading on A1?

Try 641 237 410 at 12V. This will swop over A1 and A2.

If the reading is now on A1 one half of the valve is faulty. If however the reading remains on A2 you have a tester fault.

Longespee · 2nd Apr 2010, 11:20 am

Testing with 741 226 413 A1 under spec A2 no reading (heaters in parallel)
Testing with 741 236 410 both A1 & A2 under spec but equal readings (heaters in series)

Station X · 2nd Apr 2010, 11:45 am

I'm at a loss to explain that. The only difference between the two sets of settings in post #11 is that the first has the heaters in parallel across 6V and the second has them in series across 12V.

Do the heaters actually light up when in series. If they do then both halves of the heater must be intact. If they don't than one half is open circuit.

You are changing the heater voltage when changing the thumb wheel settings?

Under spec obviously refers to gm, but do both halves of the valve draw the correct anode current at the specified grid voltage? This is a measure of emission rather than gm. 11.5mA is the quoted figure for 250V HT.

Longespee · 2nd Apr 2010, 1:25 pm

Well now I've just tried the original setting for 6v and both A1 and A2 are reading and heaters both too( dirty contacts? ), I get 7.2 Ia and mA/V 2.2 on both A1 and A2, with a test of goodness as Replace. The same readings for the 12v settings too.

Station X · 2nd Apr 2010, 1:29 pm

That sounds better. Ia and gm are both down, but the valve may work perfectly well in a circuit.

Lucien Nunes · 4th Apr 2010, 9:11 pm

It seems you are getting intermittent tester operation that changes when you adjust even the thumbwheels for the heater pins. I wonder whether doing so is disturbing the contacts on the other thumbwheels and making or breaking the circuit to other electrodes. I've been surprised in the past how well even a worn and sticky AVO circuit selector can work, but it might be worth checking. It takes a few hours to do a good job of stripping, cleaning and adjusting, for which I made a little gauge tool, but so long as you are extremely careful to preserve the order of the parts it's not difficult to overhaul.

Lucien

Longespee · 4th Apr 2010, 10:47 pm

Yes, I think this could be the case, although I do rock the wheel once its in position to make sure it is properly there. All the wheels are quite stiff and a strip down is something I will get on with soon, cheers, Sean.

1st Apr 2010, 8:22 pm	#2
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: 12BH7A but can't test If it's any help the 12BH7 has the same pinout as ECC81,82,83 etc. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

1st Apr 2010, 8:23 pm	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: 12BH7A but can't test If you are using a 6V3 heater supply, then you definitely need @@@ 22@ @@3. You would need @@@ 23@ @@0 with a 12V6 heater supply. (Thanks to Top Cap for explaining what the dials do .....) You could try 641 220 003 for the left-hand triode, and 000 226 413 for the right-hand one? See also the data sheet here. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

1st Apr 2010, 8:52 pm	#4
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	Re: 12BH7A but can't test Thanks guys, I will have a go. Could I, as the 12BH7A has a voltage of 12.3, test it as a 6V with the 741 226 413 and get a reading of goodness. Although testing earlier on 741 A1 was under spec but A2 was no reading.

1st Apr 2010, 9:35 pm	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 12BH7A but can't test When you say it says replace is this for A1, A2 or both? It's possible that one half of the heater is open circuit. This will only show up on the tester if the heaters are connected in series across a 12V supply. Unless of course both halves of the heater are O/C, which will show up on a 6V supply with the heaters in parallel. Whenever I test a new type of valve for the first time I always check the thumb switch settings against the valve data and then tick the entry in the AVO data book. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

1st Apr 2010, 9:47 pm	#6
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 12BH7A but can't test The settings 841 238 410 cannot possibly be for a double triode, as no switch is set to 6 or 7 which would be the anodes. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

1st Apr 2010, 8:04 pm	#1
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	12BH7A but can't test Perhaps I'm being a little dumb here but, I have a 12BH7A to test on my AVO mk2 and I dial up 741 226 413 (12BH7 in ed 17 AVO Data ) and it works out as REPLACE, then I realize it has an A but my list doesn't cover this one, but Ed 20 does and lists as 841 238 410, so I dial this up and get no readings at all, anyone through some light, cheers, Sean.

1st Apr 2010, 9:50 pm	#7
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	Re: 12BH7A but can't test It's all a little out of my depth really, but if I test as 741 with 6v settings then A1 is replace and A2 won't read at all, but with Edition 20 and 12BH7A settings at 841.. and 12v it won't read and all is zero. I need to sit down and go through the heater test in theory to get the whole thing to register brain wise first.

1st Apr 2010, 9:58 pm	#8
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 12BH7A but can't test The difference between the 12BH7 and 12BH7-A is explained here:- http://www.jogis-roehrenbude.de/Roeh...BH7/12BH7A.pdf Take it from me the 841 settings are wrong as they relate to testing a diode. 741 226 413 is correct for a 6V heater voltage setting. 741 236 410 is correct for a 12V heater voltage setting. An easy option would be to test the heaters for continuity with a meter on ohms range. Check betwen pins 9 & 4 and 9 & 5. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

2nd Apr 2010, 10:15 am	#9
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	Re: 12BH7A but can't test Many thanks Graham, I have tested at 741 236 410 and I can get a reading on A2, alas the two valves I have test replace. Another amendment to the AVO book I think.

2nd Apr 2010, 10:20 am	#10
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 12BH7A but can't test Are you saying there's no reading on A1? Try 641 237 410 at 12V. This will swop over A1 and A2. If the reading is now on A1 one half of the valve is faulty. If however the reading remains on A2 you have a tester fault. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

2nd Apr 2010, 11:20 am	#11
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	Re: 12BH7A but can't test Testing with 741 226 413 A1 under spec A2 no reading (heaters in parallel) Testing with 741 236 410 both A1 & A2 under spec but equal readings (heaters in series)

2nd Apr 2010, 11:45 am	#12
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 12BH7A but can't test I'm at a loss to explain that. The only difference between the two sets of settings in post #11 is that the first has the heaters in parallel across 6V and the second has them in series across 12V. Do the heaters actually light up when in series. If they do then both halves of the heater must be intact. If they don't than one half is open circuit. You are changing the heater voltage when changing the thumb wheel settings? Under spec obviously refers to gm, but do both halves of the valve draw the correct anode current at the specified grid voltage? This is a measure of emission rather than gm. 11.5mA is the quoted figure for 250V HT. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

2nd Apr 2010, 1:25 pm	#13
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	Re: 12BH7A but can't test Well now I've just tried the original setting for 6v and both A1 and A2 are reading and heaters both too( dirty contacts? ), I get 7.2 Ia and mA/V 2.2 on both A1 and A2, with a test of goodness as Replace. The same readings for the 12v settings too.

2nd Apr 2010, 1:29 pm	#14
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: 12BH7A but can't test That sounds better. Ia and gm are both down, but the valve may work perfectly well in a circuit. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

4th Apr 2010, 9:11 pm	#15
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: 12BH7A but can't test It seems you are getting intermittent tester operation that changes when you adjust even the thumbwheels for the heater pins. I wonder whether doing so is disturbing the contacts on the other thumbwheels and making or breaking the circuit to other electrodes. I've been surprised in the past how well even a worn and sticky AVO circuit selector can work, but it might be worth checking. It takes a few hours to do a good job of stripping, cleaning and adjusting, for which I made a little gauge tool, but so long as you are extremely careful to preserve the order of the parts it's not difficult to overhaul. Lucien

4th Apr 2010, 10:47 pm	#16
Longespee Pentode Join Date: Jun 2009 Location: Leominster, Herefordshire, UK. Posts: 245	Re: 12BH7A but can't test Yes, I think this could be the case, although I do rock the wheel once its in position to make sure it is properly there. All the wheels are quite stiff and a strip down is something I will get on with soon, cheers, Sean.