Valve Amplifier input level - UK Vintage Radio Repair and Restoration Discussion Forum

cassiei · 12th Aug 2020, 1:40 pm

Hi all Audio engineers.
I am after some help to modify my Jukebox Amplifier. The reason for doing so is that I have replaced the original cartridges. The original Goldring 580's only have styli in sapphire and even these are very hard to find. Also the tracking force needed is damaging the more modern stereo records installed. I trialed 2off Pickering NP/AC cartridge in the machine and was blown away with the performance, reliability and sound. No tracking issues at all along with a much lower tracking force and a diamond easy to find stylus makes this modification very worthwhile.
I know what your thinking.....what's the problem then? Well it's this. It is a little too good! The output from the Goldring is 3.3mV. As there are two, one for side A and another for side B and they are wired in series giving 6.4mV input into the amp. With the NP/AC being a Stereo cart and both channels wired in series the output is 40mV into the amp. NP/AC has a 10Mv output x 2 for each channel x 2 for two carts. (I tried wiring the 2 cats in parallel but they didn't sound as good)
Can someone think of a way to reduce this level more towards the amps requirements. As originality is important my plan is to fit another input jack with a switch to adjust between the two input levels. What would I need to do to the amps input curcuit to achieve this? I tried fitting a much larger volume pot 500K from 100K but still the volume is very loud with small movements on the pot. I don't want to compromise the sound quality if I can help it. The sound is great. It's just the control which needs sorting.
I have attached the amp schematic.
Many thanks.

Radio Wrangler · 12th Aug 2020, 3:06 pm

Don't add the voltages for the two cartridges.... at least not unless you plan pn playing both sides at once :=)

The two windings for one stereo cartridge in series seems a sensible way ro combine to get mono. Having a second cartridge in series will lose a bit of signal, but not a great deal, as it should be sitting silently with its stylus in fresh air when the other one is playing.

So you may need to arrange a bit of attenuation somewhere to make the overall volume more reasonable. Probably after the RIAA equalisation stage would do least damage to your signal to noise ratio. Attenuation just after the cartridge will worsen the noise more than you may expect... not just by dropping the signal, but also by shifting the impedance presented to the amplifier.

Easily done by changing the value of the two illegible resistors following C7. 100k into 25k. just drop the value of the 25k one until you like the level. Close enough for government work

David

cassiei · 12th Aug 2020, 11:48 pm

Hi David
Many thanks for the help here. Yes as you say only one cartridge is ever in use at any one time. Sorry about the schematic quality.....this is the only copy I have. (I have tried to clean it up a bit and reposted). Indeed information on this Jukebox is very hard to find. It's a Chantal Meteor, a British made machine. I have measured the fluctuating output from the cartridges which peaks at about 35mV. I will change as you suggest the R7 to a 25K resistor, as well as connect a substitution resistance box to R8 and see what happens. So I am adjusting the level prior to the second amplification tube. If all goes well I will fit another socket and a switch between the two.
Cheers
David

Radio Wrangler · 13th Aug 2020, 7:13 am

I don't think it's R7.

I'm having difficulties reading both value and designator.
It is the resistor from grid1 of the pentode part of the triode/pentode dual valve and goes to chassis. On this print it looks like R8 and 29K but 29K isn't a standard value. For compensating for, say double the cartridge voltage, and dropping 6dB of gain, I'd go for 12K as a first trial and then go up or down from there.

All the jukeboxes I've been involved with were Seeburgs and I got to quite like them. I did a MOSFET plug-in to replace the 2050 thyratrons in the core memory bases selector sense amplifier. 2050s were getting hard to find and the alternatives that Gordon, who owned about fifty boxes, tried getting in from the states did not work reliably.

David

mickyfinn · 14th Aug 2020, 7:51 am

David did you get my PM?
Mick

Radio Wrangler · 14th Aug 2020, 8:02 am

No, no PM received.

David

cassiei · 18th Aug 2020, 4:25 pm

Cheers David. I too am having trouble reading some of the values. Unfortunately it's all we've got to work with. So the output signal from the plate of valve V1 goes through the coupling capacitor to remove DC and then through R8 which is 100K (measured) to the grid of V2. Another resistor R8 placed after R9 is 27K (measured) goes to ground. I agree it is written as 29K not a standard resistor value. I am going to halve the 100K to 50K then adjust the 27K until a satisfactory level is achieved. Thanks

snowman_al · 18th Aug 2020, 5:58 pm

Do not lower the value of R8, that will make it louder...
Think of R8 and R9 as a potentiometer of 129k value. The 29k point is roughly a 37% 'tap'. Lower the ratio to make it quieter.
You could replace the 29K resistor with two 15k resistors in series. If you use the junction between them you have a 19% 'tap' as well as the original.
Or use a 20k in the middle position and a 10k in the lower position, that gives a 13% tap and so on.
Or put a 30k pot in there and you can adjust the tap as you wish.

cassiei · 21st Aug 2020, 5:55 pm

AL, many thanks for your post on this. Now that you have described the situation along with the explanatory drawing I have now finally got my head around it. As you say the signal is referenced to earth through a tap between the resistors. So the original set up is using a 37% tap to earth. As this is now too high I will use your two 15 ohm resistors as per your recommendation and reduce it to 19%. I think this will be very close to the desired value. Doing the job this weekend. Let you know the result.
Cheers
Ian

snowman_al · 21st Aug 2020, 6:39 pm

No problem,
Just to be sure your 15 ohms is a typo, the resistors need to be 15k ohm resistors (15,000 ohms)...

cassiei · 24th Aug 2020, 2:14 pm

Yep...a typo. Cheers again Al. I have finished the job today. Very happy with the result. Volume pot at halfway and sound is at normal listening level. Here is a sketch of what I did. Used a slide switch so that I can select between the two inputs. Easy job now to swap back to the original Cartridge should I choose to do so. But as all is good.....leaving it like it is.
Ian

12th Aug 2020, 1:40 pm	#1
cassiei Triode Join Date: Nov 2019 Location: St. Helens, Merseyside, UK. Posts: 15	Valve Amplifier input level Hi all Audio engineers. I am after some help to modify my Jukebox Amplifier. The reason for doing so is that I have replaced the original cartridges. The original Goldring 580's only have styli in sapphire and even these are very hard to find. Also the tracking force needed is damaging the more modern stereo records installed. I trialed 2off Pickering NP/AC cartridge in the machine and was blown away with the performance, reliability and sound. No tracking issues at all along with a much lower tracking force and a diamond easy to find stylus makes this modification very worthwhile. I know what your thinking.....what's the problem then? Well it's this. It is a little too good! The output from the Goldring is 3.3mV. As there are two, one for side A and another for side B and they are wired in series giving 6.4mV input into the amp. With the NP/AC being a Stereo cart and both channels wired in series the output is 40mV into the amp. NP/AC has a 10Mv output x 2 for each channel x 2 for two carts. (I tried wiring the 2 cats in parallel but they didn't sound as good) Can someone think of a way to reduce this level more towards the amps requirements. As originality is important my plan is to fit another input jack with a switch to adjust between the two input levels. What would I need to do to the amps input curcuit to achieve this? I tried fitting a much larger volume pot 500K from 100K but still the volume is very loud with small movements on the pot. I don't want to compromise the sound quality if I can help it. The sound is great. It's just the control which needs sorting. I have attached the amp schematic. Many thanks. Attached Thumbnails

12th Aug 2020, 3:06 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Valve Amplifier input level Don't add the voltages for the two cartridges.... at least not unless you plan pn playing both sides at once :=) The two windings for one stereo cartridge in series seems a sensible way ro combine to get mono. Having a second cartridge in series will lose a bit of signal, but not a great deal, as it should be sitting silently with its stylus in fresh air when the other one is playing. So you may need to arrange a bit of attenuation somewhere to make the overall volume more reasonable. Probably after the RIAA equalisation stage would do least damage to your signal to noise ratio. Attenuation just after the cartridge will worsen the noise more than you may expect... not just by dropping the signal, but also by shifting the impedance presented to the amplifier. Easily done by changing the value of the two illegible resistors following C7. 100k into 25k. just drop the value of the 25k one until you like the level. Close enough for government work David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

12th Aug 2020, 11:48 pm	#3
cassiei Triode Join Date: Nov 2019 Location: St. Helens, Merseyside, UK. Posts: 15	Re: Valve Amplifier input level Hi David Many thanks for the help here. Yes as you say only one cartridge is ever in use at any one time. Sorry about the schematic quality.....this is the only copy I have. (I have tried to clean it up a bit and reposted). Indeed information on this Jukebox is very hard to find. It's a Chantal Meteor, a British made machine. I have measured the fluctuating output from the cartridges which peaks at about 35mV. I will change as you suggest the R7 to a 25K resistor, as well as connect a substitution resistance box to R8 and see what happens. So I am adjusting the level prior to the second amplification tube. If all goes well I will fit another socket and a switch between the two. Cheers David Attached Thumbnails

13th Aug 2020, 7:13 am	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Valve Amplifier input level I don't think it's R7. I'm having difficulties reading both value and designator. It is the resistor from grid1 of the pentode part of the triode/pentode dual valve and goes to chassis. On this print it looks like R8 and 29K but 29K isn't a standard value. For compensating for, say double the cartridge voltage, and dropping 6dB of gain, I'd go for 12K as a first trial and then go up or down from there. All the jukeboxes I've been involved with were Seeburgs and I got to quite like them. I did a MOSFET plug-in to replace the 2050 thyratrons in the core memory bases selector sense amplifier. 2050s were getting hard to find and the alternatives that Gordon, who owned about fifty boxes, tried getting in from the states did not work reliably. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

14th Aug 2020, 7:51 am	#5
mickyfinn Hexode Join Date: Dec 2014 Location: Scarborough, North Yorkshire, UK. Posts: 473	Re: Valve Amplifier input level David did you get my PM? Mick

14th Aug 2020, 8:02 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: Valve Amplifier input level No, no PM received. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

18th Aug 2020, 4:25 pm	#7
cassiei Triode Join Date: Nov 2019 Location: St. Helens, Merseyside, UK. Posts: 15	Re: Valve Amplifier input level Cheers David. I too am having trouble reading some of the values. Unfortunately it's all we've got to work with. So the output signal from the plate of valve V1 goes through the coupling capacitor to remove DC and then through R8 which is 100K (measured) to the grid of V2. Another resistor R8 placed after R9 is 27K (measured) goes to ground. I agree it is written as 29K not a standard resistor value. I am going to halve the 100K to 50K then adjust the 27K until a satisfactory level is achieved. Thanks

18th Aug 2020, 5:58 pm	#8
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: Valve Amplifier input level Do not lower the value of R8, that will make it louder... Think of R8 and R9 as a potentiometer of 129k value. The 29k point is roughly a 37% 'tap'. Lower the ratio to make it quieter. You could replace the 29K resistor with two 15k resistors in series. If you use the junction between them you have a 19% 'tap' as well as the original. Or use a 20k in the middle position and a 10k in the lower position, that gives a 13% tap and so on. Or put a 30k pot in there and you can adjust the tap as you wish. Attached Thumbnails __________________ Never Leave Well Enough Alone...

21st Aug 2020, 5:55 pm	#9
cassiei Triode Join Date: Nov 2019 Location: St. Helens, Merseyside, UK. Posts: 15	Re: Valve Amplifier input level AL, many thanks for your post on this. Now that you have described the situation along with the explanatory drawing I have now finally got my head around it. As you say the signal is referenced to earth through a tap between the resistors. So the original set up is using a 37% tap to earth. As this is now too high I will use your two 15 ohm resistors as per your recommendation and reduce it to 19%. I think this will be very close to the desired value. Doing the job this weekend. Let you know the result. Cheers Ian

21st Aug 2020, 6:39 pm	#10
snowman_al Octode Join Date: Dec 2018 Location: Llandeilo, West Wales, UK. Posts: 1,092	Re: Valve Amplifier input level No problem, Just to be sure your 15 ohms is a typo, the resistors need to be 15k ohm resistors (15,000 ohms)... __________________ Never Leave Well Enough Alone...

24th Aug 2020, 2:14 pm	#11
cassiei Triode Join Date: Nov 2019 Location: St. Helens, Merseyside, UK. Posts: 15	Re: Valve Amplifier input level Yep...a typo. Cheers again Al. I have finished the job today. Very happy with the result. Volume pot at halfway and sound is at normal listening level. Here is a sketch of what I did. Used a slide switch so that I can select between the two inputs. Easy job now to swap back to the original Cartridge should I choose to do so. But as all is good.....leaving it like it is. Ian Attached Thumbnails