What am I missing with this amp ?

Burning Fingers

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I am working on a 1960's 40 watt Goldentone amp..a model 1755 ..aka 40 watt Slimline combo.

The complaint is that it is too shrill and lacking in mids and bass on both channels.

Ok... so that means the problem in downstream from the preamp section as only the PI and power tubes are used by both channels.

I have tried new power tubes...no change.

I tried new preamp tubes...a slight change in both channels.

Checked the speakers to find that there are two 8 ohm speakers in parallel .

Checked the specs of the output transformer ( A&R 2700) to find that it expects an 8 ohm load so disconnected one speaker and powered it up.

A very loud squeal resulted so I reconnected the other speaker and no squeal.

Tried with another 8 ohm speaker and yes...squeal...tried another 4 ohm speaker and no squeal.

There is negative feedback in the amp but would have thought that having the output primaries reversed would cause positive feedback and squealing regardless of the speaker impedance.

I'm starting to think that the output transformer has a fault...but the fault may well be in my head due to chemo brain fog.

Any ideas you wizards of the tubes ?

Cheers

1755goldentone.gif
 
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trev333

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The only real issue with mine is scratchy pots.... when I got it there was only one tube. it took me a few tubes to get one that matched the orig OK.. red plating old tubes it such fun...

all good now..

I know a guy up here that got mine going who is a whiz with tube circuits.

G'Tone 'Verbmaster + Vase.jpg


Goldentone 6DQ6A Valves - smaller1.jpg
 

Burning Fingers

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Just to make it more interesting...I found a 0.001uf capacitor connected in series with the feedback resistor....there should not be a capacitor in the negative feedback circuit

I put a jumper cable across the capacitor and Boom...the mids and lows returned big time and the amp farted out a halfway on the volume dial when any low mid to low notes were played.

I can forget about this being a quick repair lol.
 

trev333

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The tubes were originally horizontal hold TV tubes... with OS stuff hard to get back then they used what they could...

I don't think these tubes were used as audio power tubes before... and worked out fine.
 

Burning Fingers

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Has anyone worked on the amp before ?
Yes...lots of new shiny solder joints but no new components that I can see ( apart from the addition of the capacitor in the negative feedback circuit.

I wish that people who make changes to amps would document those changes and the reason for those changes and tape the documentation inside the cab somewhere.
 

nickmm

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Yes the voltages are ok
A tech in Sydney since passed worked on my friends. He converted the power section to 6l6 his brother was the Builder of Woogie amps.

The power tubes in these things are TV tubes.
I'd be checking around the that weird bias circuit.
Do you have a spare output transformer to test?
 

Burning Fingers

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A tech in Sydney since passed worked on my friends. He converted the power section to 6l6 his brother was the Builder of Woogie amps.

The power tubes in these things are TV tubes.
I'd be checking around the that weird bias circuit.
Do you have a spare output transformer to test?
Luckily I have another output transformer..not the same model but close enough and from another Goldie I stripped for parts.

I have plenty of NOS power tubes for Goldentone amps... I buy them whenever they are going for a low price.
 

nickmm

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Luckily I have another output transformer..not the same model but close enough and from another Goldie I stripped for parts.

I have plenty of NOS power tubes for Goldentone amps... I buy them whenever they are going for a low price.

I like to help more but don't know enough about 6DQ6A and that circuit for the second plate with 180 volts.
 

Scooby9261

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I got this kind of squeal when I was installing a Negative Feedback switch on one of my amps a few years ago and the wires on the switch were backwards. All i did was reverse the wires and it stopped. Could be something that has to do with this if you have a NFB pot or switch. Also you said it stops with 4ohm speaker, I think the OT has the wrong wire running to the input jack. It may say 8ohm, but some transformers have 4, 8 and 16ohm taps. Could be someone ran the wrong wire ?

Just throwing it out there
 

Dacious

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I've owned a couple of these. A cathode biased Bassmaster 20 with 6dq6A and a 40 with fixed bias 6DQ6Bs.

The Goldies run very conservative volts. It's a long time ago but IIRC the 20 had 350 volts on the plates and the 40 375 volts. 6DQ6s can run 700 volts......

They do run max 50% grid voltage. What that does to voltage swing and frequency response? Hifi types like them because they are very linear - for guitar amps that can mean (I think) they seem anodyne compared to typical Fenders which distort all over the place. Preamps, power amps, output transformers, speakers.

The whole preamp on these amps is run very mild. It's not uncommon to see 12AU7s as phase inverters. Possibly because tubes in Australia are rare and expensive. Every Goldie, Moody, Eminar, Diason, Electravox, Maton, I've looked at came with most if not all their original tubes. Usually AWV or Phillips Miniwatt. They just don't wear out.

I think Australian amp developers were looking for clean and smooth. Their early echoes are subtle. The Reverbmaster is much better at a surfy tremolo and reverb. Not maybe as good as a Twin Reverb.

That means until you hit the saturation limits of a transformer you get not much natural distortion. Preamps stay very clean. I've had a 35 watt Moody with topcaps and one with EL34s to work on. The later cream mid 60s one was a bit more Marshall JTM like. Later Eminars started to go more Marshall like.

You can goose them up. But according to Aussie Gear Heads that loses the sweetness. You can see why bands like Masters Apprentices, Daddy Cool and Easybeats went Vox or Fender. Hi Hi Honey Hi film clip shows Wilson and Hannaford playing Blackface Super Reverbs in 73.

I've got a Vadis in bits in my garage. It's the III hybrid SS preamp version of the EL34 40 watt head. Clean clean clean. Ostensibly it's a Bassman clone with a rhythm and bass channel.

Got no bassman BRRRRRRR mid overdrive at all. I'm going to convert it to a Fendery preamp with reverb.
 
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Burning Fingers

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I got this kind of squeal when I was installing a Negative Feedback switch on one of my amps a few years ago and the wires on the switch were backwards. All i did was reverse the wires and it stopped. Could be something that has to do with this if you have a NFB pot or switch. Also you said it stops with 4ohm speaker, I think the OT has the wrong wire running to the input jack. It may say 8ohm, but some transformers have 4, 8 and 16ohm taps. Could be someone ran the wrong wire ?

Just throwing it out there
The transformer only has an 8 ohm secondary and no NFB switch or pot so there's no mis-wiring of that involved.
 

Gunny

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I once had a vintage Gibson with those tone limitations. I found it had a 0.005 uF cap in the signal line. I looked at the schematic for your amp and saw the 0.001 uF going into the phase inverter and said 'that's the problem'. You discovered the same component and remedied it.
 

Burning Fingers

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I once had a vintage Gibson with those tone limitations. I found it had a 0.005 uF cap in the signal line. I looked at the schematic for your amp and saw the 0.001 uF going into the phase inverter and said 'that's the problem'. You discovered the same component and remedied it.
Yep I found a 0.001 uf cap alright but this one is extra to what is on the schematic and is in the NFB line between the output socket tip and the 22k NFB resistor...removing the cap from the NFB line solved the poor mid/bass response so. I will double check that coupling cap that feeds the PI and see if the schematic is correct. ( none of the Goldentone schematics available are manufacture schematics...all have been reverse engineered by Goldentone owners or techs.)

The farting out that's there now that I have removed that offending NFB cap is probably due to the power supply caps being well over due for replacement...could that be why the extra cap was put into the NFB...to lower bass response and stop the farting out rather than recapping the the power supply ?.

These amps really do sound Golden when working properly but they are now around 60 years old and most have had a few "repairs/modifications" over the years and it can be time consuming to figure out what has been done to them and why .
 

myfenderissues

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I am working on a 1960's 40 watt Goldentone amp..a model 1755 ..aka 40 watt Slimline combo.

The complaint is that it is too shrill and lacking in mids and bass on both channels.

Ok... so that means the problem in downstream from the preamp section as only the PI and power tubes are used by both channels.

I have tried new power tubes...no change.

I tried new preamp tubes...a slight change in both channels.

Checked the speakers to find that there are two 8 ohm speakers in parallel .

Checked the specs of the output transformer ( A&R 2700) to find that it expects an 8 ohm load so disconnected one speaker and powered it up.

A very loud squeal resulted so I reconnected the other speaker and no squeal.

Tried with another 8 ohm speaker and yes...squeal...tried another 4 ohm speaker and no squeal.

There is negative feedback in the amp but would have thought that having the output primaries reversed would cause positive feedback and squealing regardless of the speaker impedance.

I'm starting to think that the output transformer has a fault...but the fault may well be in my head due to chemo brain fog.

Any ideas you wizards of the tubes ?

Cheers

View attachment 1009626

the cathode bypass cap at 25 microfards on the first preamp tube is pretty small. fender used it on amps from 1960 on but try a larger cap, even up to 350 mf's. and put in 68 k resistors at the input jacks. after that, the 560 pf sized bright caps in the tone stack are large. perhaps the amp was voiced back then for using hollow body guitars with darker sounding pickups that needed more treble boost at the amp to sound good? that 25mf bypass cap, even though people say by math that it lets all the frequencies for a 6 string guitar through, will cut a lot of bass and midrange out. the late 50's bassman had a 350mf cap there. just to experiment, i've used a 25 mf cap there on a bassman, and it cuts the low and mids out a lot.
 

Burning Fingers

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I've owned a couple of these. A cathode biased Bassmaster 20 with 6dq6A and a 40 with fixed bias 6DQ6Bs.

The Goldies run very conservative volts. It's a long time ago but IIRC the 20 had 350 volts on the plates and the 40 375 volts. 6DQ6s can run 700 volts......

They do run max 50% grid voltage. What that does to voltage swing and frequency response? Hifi types like them because they are very linear - for guitar amps that can mean (I think) they seem anodyne compared to typical Fenders which distort all over the place. Preamps, power amps, output transformers, speakers.

The whole preamp on these amps is run very mild. It's not uncommon to see 12AU7s as phase inverters. Possibly because tubes in Australia are rare and expensive. Every Goldie, Moody, Eminar, Diason, Electravox, Maton, I've looked at came with most if not all their original tubes. Usually AWV or Phillips Miniwatt. They just don't wear out.

I think Australian amp developers were looking for clean and smooth. Their early echoes are subtle. The Reverbmaster is much better at a surfy tremolo and reverb. Not maybe as good as a Twin Reverb.

That means until you hit the saturation limits of a transformer you get not much natural distortion. Preamps stay very clean. I've had a 35 watt Moody with topcaps and one with EL34s to work on. The later cream mid 60s one was a bit more Marshall JTM like. Later Eminars started to go more Marshall like.

You can goose them up. But according to Aussie Gear Heads that loses the sweetness. You can see why bands like Masters Apprentices, Daddy Cool and Easybeats went Vox or Fender. Hi Hi Honey Hi film clip shows Wilson and Hannaford playing Blackface Super Reverbs in 73.

I've got a Vadis in bits in my garage. It's the III hybrid SS preamp version of the EL34 40 watt head. Clean clean clean. Ostensibly it's a Bassman clone with a rhythm and bass channel.

Got no bassman BRRRRRRR mid overdrive at all. I'm going to convert it to a Fendery preamp with reverb.
I have worked on at least 40 goldentones in the past 5 years and have a couple of my own...usually any repairs are quick and easy but now and again I get thrown a curve ball like with this amp.

I would never mess with a Goldie to get more power.. what a crime that would be..any way most of the Goldies I work on belong to collectors who might use them once a week at the most and want to keep them as original as possible. The transformers are so big that you really have to drive the amps hard to get any breakup.

If you want a powerful aussie amp then an Enimar 100 watt amp will peel the paint off your walls.
 

nickmm

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Yep I found a 0.001 uf cap alright but this one is extra to what is on the schematic and is in the NFB line between the output socket tip and the 22k NFB resistor...removing the cap from the NFB line solved the poor mid/bass response so. I will double check that coupling cap that feeds the PI and see if the schematic is correct. ( none of the Goldentone schematics available are manufacture schematics...all have been reverse engineered by Goldentone owners or techs.)

The farting out that's there now that I have removed that offending NFB cap is probably due to the power supply caps being well over due for replacement...could that be why the extra cap was put into the NFB...to lower bass response and stop the farting out rather than recapping the the power supply ?.

These amps really do sound Golden when working properly but they are now around 60 years old and most have had a few "repairs/modifications" over the years and it can be time consuming to figure out what has been done to them and why .
My first teaching gig had one of these as the schools amp.
I wanted a Boogie at the time. They have a unique sound.
To add one of my student's Granddad was the Vadis (I think or Moody) company owner. He didn't know much about his business.
 




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