Cathode follower - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 7th Apr 2018, 5:30 pm

Hi all

got an early 90's Korg era AC30 here that's got a feeble output on the top boost channel. The signal is good at the anode/grid junction (R55) but it's super quiet on the cathode follower (R8) feeding the EQ section. Am I right to expect a similar signal voltage here as on the grid of that triode?

The 12AX7 is new, the 1st anode voltage is 208V, the second 305V, R8 reads good at 56K

Many thanks

ukcol · 7th Apr 2018, 6:41 pm

I am just speculating here. Perhaps C36 is open circuit, resulting in the gain of the second triode is being reduced by loads of negative feedback?

ITAM805 · 7th Apr 2018, 8:00 pm

Thanks Colin, I piggy backed a 47uF across C36 but still the same result. I though that a cathode follower circuit has a little less than unity gain, if that's so then where's the signal going?

ms660 · 7th Apr 2018, 8:05 pm

What's the cathode followers cathode voltage?

Lawrence.

ITAM805 · 7th Apr 2018, 8:15 pm

Hi Lawrence, well it was 6.5V but has slowly crept up to 13V after a few minutes

It stabilized at 14.3V

ms660 · 7th Apr 2018, 8:47 pm

I would have thought it should be slightly higher than the grid voltage.

Lawrence.

Cobaltblue · 7th Apr 2018, 8:47 pm

I would expect the Cathode of the second part to be almost the same voltage as anode of the first.

Around 180V

Looks as if there is a problem either with the valve the socket or some parts around it.

Cheers

Mike T

ITAM805 · 7th Apr 2018, 9:01 pm

Lawrence, Mike, thanks. Well at least I was on the right track. I'll check out the socket etc, and see what I can find...

Biggles · 7th Apr 2018, 10:02 pm

Seems odd that there is so much drift before it settles. Maybe the thing has some HF oscillation or instability going on, although I would have thought that was noticeable when you checked the signal levels with a scope.
Alan.

ITAM805 · 8th Apr 2018, 12:02 am

Well I tried everything but couldn't find owt wrong? I thought it cant be the valve can it, it's a brand new boxed Electro-Harmonix ? So nothing to lose I tried an old one from the stash - and back came the volume! As Harry Hill used to say, what are chances of that happening?

paulsherwin · 8th Apr 2018, 12:50 am

I suppose there have always been DOA new valves.

G8UWM-MildMartin · 8th Apr 2018, 2:07 am

Also, please take into account the thread:
https://www.vintage-radio.net/forum/...d.php?t=138636
as to what happened to a number of tested, stable, known good valves that went through various postal systems!

John10b · 8th Apr 2018, 4:26 pm

As matter of interest, did you measure the voltages after successfully getting it going?
Cheers
John

ITAM805 · 8th Apr 2018, 9:00 pm

Quote:

did you measure the voltages after successfully getting it going?

Hi John, yes 188V across R8

BTW, I tested the faulty EH valve in question and there was zero emission on one of the triodes, so how it passed any signal is a miracle!

joebog1 · 9th Apr 2018, 12:38 am

I dont like bagging various products, but I wont even allow EH valves into the house!!
I have had many more than one dead on arrival, several flashing over at very modest operating voltages and generally very noisy. I once had to buy 20 KT88's to get four to operate in a Parry Cath-amp made in japan. WITHOUT red plating!!!

Joe

7th Apr 2018, 5:30 pm	#1
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Cathode follower Hi all got an early 90's Korg era AC30 here that's got a feeble output on the top boost channel. The signal is good at the anode/grid junction (R55) but it's super quiet on the cathode follower (R8) feeding the EQ section. Am I right to expect a similar signal voltage here as on the grid of that triode? The 12AX7 is new, the 1st anode voltage is 208V, the second 305V, R8 reads good at 56K Many thanks Attached Thumbnails

7th Apr 2018, 6:41 pm	#2
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Cathode follower I am just speculating here. Perhaps C36 is open circuit, resulting in the gain of the second triode is being reduced by loads of negative feedback?

7th Apr 2018, 8:00 pm	#3
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Cathode follower Thanks Colin, I piggy backed a 47uF across C36 but still the same result. I though that a cathode follower circuit has a little less than unity gain, if that's so then where's the signal going?

7th Apr 2018, 8:05 pm	#4
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Cathode follower What's the cathode followers cathode voltage? Lawrence.

7th Apr 2018, 8:15 pm	#5
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Cathode follower Hi Lawrence, well it was 6.5V but has slowly crept up to 13V after a few minutes It stabilized at 14.3V Last edited by ITAM805; 7th Apr 2018 at 8:24 pm.

7th Apr 2018, 8:47 pm	#6
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Cathode follower I would have thought it should be slightly higher than the grid voltage. Lawrence.

7th Apr 2018, 8:47 pm	#7
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,879	Re: Cathode follower I would expect the Cathode of the second part to be almost the same voltage as anode of the first. Around 180V Looks as if there is a problem either with the valve the socket or some parts around it. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

7th Apr 2018, 9:01 pm	#8
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Cathode follower Lawrence, Mike, thanks. Well at least I was on the right track. I'll check out the socket etc, and see what I can find...

7th Apr 2018, 10:02 pm	#9
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Cathode follower Seems odd that there is so much drift before it settles. Maybe the thing has some HF oscillation or instability going on, although I would have thought that was noticeable when you checked the signal levels with a scope. Alan.

8th Apr 2018, 12:02 am	#10
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Cathode follower Well I tried everything but couldn't find owt wrong? I thought it cant be the valve can it, it's a brand new boxed Electro-Harmonix ? So nothing to lose I tried an old one from the stash - and back came the volume! As Harry Hill used to say, what are chances of that happening?

8th Apr 2018, 12:50 am	#11
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Cathode follower I suppose there have always been DOA new valves.

8th Apr 2018, 2:07 am	#12
G8UWM-MildMartin Heptode Join Date: Jan 2004 Location: Stockport, Greater Manchester, UK. Posts: 827	Re: Cathode follower Also, please take into account the thread: https://www.vintage-radio.net/forum/...d.php?t=138636 as to what happened to a number of tested, stable, known good valves that went through various postal systems!

8th Apr 2018, 4:26 pm	#13
John10b Nonode Join Date: Nov 2016 Location: Aberaeron, Ceredigion, Wales, UK. Posts: 2,887	Re: Cathode follower As matter of interest, did you measure the voltages after successfully getting it going? Cheers John

9th Apr 2018, 12:38 am	#15
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Cathode follower I dont like bagging various products, but I wont even allow EH valves into the house!! I have had many more than one dead on arrival, several flashing over at very modest operating voltages and generally very noisy. I once had to buy 20 KT88's to get four to operate in a Parry Cath-amp made in japan. WITHOUT red plating!!! Joe