Michaelson & Austin TVA-1 driver PCB etc. - UK Vintage Radio Repair and Restoration Discussion Forum

leakstereo20 · 10th Apr 2016, 4:18 pm

For repair and troubleshooting of the TVA-1 valve Amplifier from Michaelson & Austin , I have written a diagram of the wiring seen from the bottom of the amplifier.
As far as I know it is the first time that both the driver PCB, bias PCB, component list and hardwiring diagram is available for TVA-1.

Schematic taken from TVA-1 serial no. 553

Note:
The 2016 drawing is taken from Michaelson & Austin serial no.16 and there may be changes in component values at later amplifier versions.

rgds

Herald1360 · 10th Apr 2016, 7:26 pm

First thought was why a 200V odd grid bias supply? Then I noticed the LTP input/phase splitter stage.

Interesting......

Radio Wrangler · 10th Apr 2016, 8:46 pm

It's just due to the voltage they got by hanging a half-wave rectifier off the input to the main HT bridge rectifier. It gets potted down to a hundred volts for the bottom of the long tail resistors.

Did you spot the cheeky little capacitor C13 which gives a little phase correction at the HF end to fix the high frequency stability?

David

Herald1360 · 10th Apr 2016, 11:02 pm

Didn't spot that, but can you make sense of the voltage tappings on the HT secondary?

I did think "voltage doubler" for the bias supply, but it's actually weirder than that.

leakstereo20 · 11th Apr 2016, 7:36 pm

The bias PCB in TVA-1 looks for the sake of completeness as follows.

....and two components missing in the component list:

1. C20 0,47uF / 630V MKT 1813 on bias PCB.
2. R33, 15K on bias PCB

rgds

leakstereo20 · 18th Apr 2016, 7:09 am

If anyone knows?

I would like to know approximately how many copies Michaelson & Austin made of the well known TVA-1 poweramplifier?

From some japanese sites I found serial numbers above 2100 pcs.

Another question is the size of the main transformer (AC). On the erly TVA-1 versions the main transformer is smaller (lower) than the outputtransformer. Up to serial no. 553 the main AC transformer is still the smaller type. At which serial number changes the size of the main transformer on TVA-1?

At the point of ( larger main transformer) the TVA-1 serial number where the heater current was increased, the secondary tapping points on the main transformer also changed.
The smaller main transformer had 0 - 400 - 440Vac on the secondary and 0V - CT - 6,3Vac for all heaters (thats it). ......minor change on serial no. 553 where 0 - 370Vac was added....

The later versions (larger size) of main transformers had 190 - 0 - 190Vac , 0 - 12Vac (KT88)- 100Vac(bias) and 6,3Vac for ECC81,83

Any information about TVA-1 production figures and changes would be highly appreciated.

Rgds.

10th Apr 2016, 4:18 pm	#1
leakstereo20 Retired Dormant Member Join Date: Oct 2011 Location: Herlev, Denmark Posts: 94	Michaelson & Austin TVA-1 driver PCB etc. For repair and troubleshooting of the TVA-1 valve Amplifier from Michaelson & Austin , I have written a diagram of the wiring seen from the bottom of the amplifier. As far as I know it is the first time that both the driver PCB, bias PCB, component list and hardwiring diagram is available for TVA-1. Schematic taken from TVA-1 serial no. 553 Note: The 2016 drawing is taken from Michaelson & Austin serial no.16 and there may be changes in component values at later amplifier versions. rgds Attached Thumbnails

10th Apr 2016, 7:26 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Michaelson & Austin TVA-1 driver PCB etc. First thought was why a 200V odd grid bias supply? Then I noticed the LTP input/phase splitter stage. Interesting...... __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Apr 2016, 8:46 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,870	Re: Michaelson & Austin TVA-1 driver PCB etc. It's just due to the voltage they got by hanging a half-wave rectifier off the input to the main HT bridge rectifier. It gets potted down to a hundred volts for the bottom of the long tail resistors. Did you spot the cheeky little capacitor C13 which gives a little phase correction at the HF end to fix the high frequency stability? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

10th Apr 2016, 11:02 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Michaelson & Austin TVA-1 driver PCB etc. Didn't spot that, but can you make sense of the voltage tappings on the HT secondary? I did think "voltage doubler" for the bias supply, but it's actually weirder than that. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

11th Apr 2016, 7:36 pm	#5
leakstereo20 Retired Dormant Member Join Date: Oct 2011 Location: Herlev, Denmark Posts: 94	Re: Michaelson & Austin TVA-1 driver PCB etc. The bias PCB in TVA-1 looks for the sake of completeness as follows. ....and two components missing in the component list: 1. C20 0,47uF / 630V MKT 1813 on bias PCB. 2. R33, 15K on bias PCB rgds Attached Thumbnails

18th Apr 2016, 7:09 am	#6
leakstereo20 Retired Dormant Member Join Date: Oct 2011 Location: Herlev, Denmark Posts: 94	Re: Michaelson & Austin TVA-1 driver PCB etc. If anyone knows? I would like to know approximately how many copies Michaelson & Austin made of the well known TVA-1 poweramplifier? From some japanese sites I found serial numbers above 2100 pcs. Another question is the size of the main transformer (AC). On the erly TVA-1 versions the main transformer is smaller (lower) than the outputtransformer. Up to serial no. 553 the main AC transformer is still the smaller type. At which serial number changes the size of the main transformer on TVA-1? At the point of ( larger main transformer) the TVA-1 serial number where the heater current was increased, the secondary tapping points on the main transformer also changed. The smaller main transformer had 0 - 400 - 440Vac on the secondary and 0V - CT - 6,3Vac for all heaters (thats it). ......minor change on serial no. 553 where 0 - 370Vac was added.... The later versions (larger size) of main transformers had 190 - 0 - 190Vac , 0 - 12Vac (KT88)- 100Vac(bias) and 6,3Vac for ECC81,83 Any information about TVA-1 production figures and changes would be highly appreciated. Rgds.