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Discussion Starter · #1 ·
I have a YRM-1 reverb master and its schematic which I can read fine..
I would like to try some of the recommended mods but my problem is identifying the actual components on the chassis as they are no labelled.


Parkhead wrote
"There are two simple ways to add gain that do not disturb the other features of the amp.

#1 cathode resistors
if you look at the schematic of the amp you are looking for v1a and v1b
the bias for that tube is set using a 1500ohm resistor on each half of the tube PINS 3 and 8
you could probably drop that resistor value down to about 820 ohm giving a bump in gain
FWIW I would only do the cathode resistor on v1b (the output of the tone stack ) and I would use 1k as the value
this will give the amp a little more internal gain range ...without losing what you already like

#2 plate load resistor
if you follow v1a you will notice its power supply feed is a 100k resistor ... this is also known as the plate load resistor
you can change its value to 220k as used in the guitar mate ....
while this adds apparent gain by sending more signal throught the attached coupling cap to the next stage
there is way more going on ... A... you are dropping the operating voltage of the tube
B... you lose some headroom in that tube C... the tube has more apparent growl
I like a little growl so I would change the plate load on V1a to 220k ...if you like Fender style headroom skip this mod

both of these tweaks are fairly subtle and would result in what you describe ... the amp sounds the same and has all stock features
but has a little more gain incorporated into the sweep of the volume control so the amp is a little more fun to play
in some ways the effect is similar to a slight boost but since it built into the amp the amp will still respond to a booster pedal
your friends will think your playing has improved, and your guitar is made of magic vintage wood

do not do this if you don't know what you are doing

do the tweaks stepwise and listen and evaluate the changes by playing as you go

the 100k plate load is in line with the power supply so you can get zapped

#3 if and when you decide to recap the amp with new power filters, probably more important than new tubes in beasts of this age
consider lowering the filter values in the pre amp ... this will also enhance the growl and touch of the amp
the 40mfd cans that filter the pre amp sections can be replaced with filters as low as 20mfd
the first B+ filter should stay 40mfd however in an amp like this I often double it to 80mfd (both sides of a paired can ) if I am lowering the filters in the rest of the amp
the raised B+ filter improves the initial noise reduction effect and the bass output of the power amp ... lowering the pre amp filters improves the touch of the pre amp
and tightens the pre amp bottom end (this filter scheme is a bit radical and you won't see it described elsewhere.. but it sounds a lot more Tweed/Plexi )

c6 is already 10uf in these make sure you stay at that value and do not add capacitance killing the tone you like
also r42 is a big sandbox 470ohm choke/ screen resistor can be changed out to 2 or 3k to better protect the output tubes
and shave a hair off the power output, the power amp becomes warmer and the screens react a little more slowly protecting the tubes and almost imperceptibly cutting output the power
(in reality you hear this as less loud brash edginess in the power amp) "

If someone could label the components (in bold ) mentioned on the picture I would be so grateful

Thanks


Engine Electrical wiring Auto part Vehicle
 

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I would like to try some of the recommended mods but my problem is identifying the actual components on the chassis as they are no labelled.
No disrespect intended, but I really think you should learn how to trace the circuit to the areas you want to mod. It sounds as if you don't really know what you are doing, and that could be a recipe for disaster. Figure it out, mark the picture yourself and post the marked up picture, then we can tell you if you are right or wrong. That way you'll learn how to do it.
As to the mods, a few comments.
1500ohm resistor on each half of the tube PINS 3 and 8
you could probably drop that resistor value down to about 820 ohm giving a bump in gain
While changing the resistor will change the operating point of the tube, and lower the plate voltage. It will not change the gain. Those resistors are bypassed by capacitors. This looks to the tube like a dead short at AC signal, while leaving the DC operating point the same. it will however lower plate voltage which will make the tube overload quicker for more grunt.
Changing out the 100k plate resistor to 220k will change the gain (about 3 dB) and will lower plate voltage to about 100V, so less headroom. Changing both cathode, and plate resistors will quite substantially cut headroom. Probably leaving you with about 75V on the plate.
also r42 is a big sandbox 470ohm choke/ screen resistor can be changed out to 2 or 3k to better protect the output tube
This is an interesting change. It will help with tube longevity, and may make a rather interesting change to the tone, especially near its power limits. Just use a good quality high wattage (10 watt min)) resistor as it is going to drop a fair bit of voltage.

As always - Safety first. Check and double check to make sure voltages are drain off, and don't work with the amp plugged in!
 

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Discussion Starter · #3 ·
If I could identify the components I would not have posted the question - indeed I don't know what I am doing on this amp despite a degree in engineering :D

On my other amp - a Fender Super Champ x2 the components are labelled on the circuit board.
I have no problem putting in the work - some guidance as to how to identify which pins are what on the tubes would be a great help

Thanks for the comments on the mods.
 

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As always - Safety first. Check and double check to make sure voltages are drained off, and don't work with the amp plugged in!
No disrespect intended, but I really think you should learn how to trace the circuit to the areas you want to mod. It sounds as if you don't really know what you are doing, and that could be a recipe for disaster. Figure it out, mark the picture yourself and post the marked up picture, then we can tell you if you are right or wrong. That way you'll learn how to do it.
As to the mods, a few comments.
@Sirtaz Ranauta Welcome to the forum.

@dtsaudio is a skilled and knowledgable amp tech. He is giving you excellent advice. I am just stating the same thing all over again as part of my welcome and for emphasis. I am not an amp tech, I just like to read about electronics and do simple guitar wiring.

Do you know how to identify and drain capacitors? These can hold lethal voltages in them!!!!

Read up on the safety aspects of working on a tube amp before you touch anything inside the chassis.

PLEASE BE CAREFUL!!!

Looking forward to seeing your drawing and to following this thread.

Cheers

Dave

Edit: @Sirtaz Ranauta You posted while I was typing.
Given that you wrote "Indeed, I don't know what I am doing on this amp" ...I will not delete my post.
 

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Discussion Starter · #5 ·
@Sirtaz Ranauta Welcome to the forum.

@dtsaudio is a skilled and knowledgable amp tech. He is giving you excellent advice. I am just stating the same thing all over again as part of my welcome and for emphasis. I am not an amp tech, I just like to read about electronics and do simple guitar wiring.

Do you know how to identify and drain capacitors? These can hold lethal voltages in them!!!!

Read up on the safety aspects of working on a tube amp before you touch anything inside the chassis.

PLEASE BE CAREFUL!!!

Looking forward to seeing your drawing and to following this thread.

Cheers

Dave

Hello - thanks for the welcome :)

Yes I am aware of the risk from the stored charge in the large capacitors and how to discharge them.

I was just hoping for a leg up to save some time.
 

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Hello - thanks for the welcome :)

Yes I am aware of the risk from the stored charge in the large capacitors and how to discharge them.

I was just hoping for a leg up to save some time.
Please understand that I did not intend to offend your knowledge/experience in any way. We are a caring community in this forum.

I hope the amp mods work out well for you.

Cheers

Dave
 

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Hi...I think what's required is for you to take the time to follow the leads as they go through the amp starting either at the power end or input end. It's a relatively simple design and easy to do. It gets easier with practice. I get amps in that have no schematics available and guess what? They all share similar layouts.
It's best for you to discover what corresponds with the schematic as it will give you a clear idea of what goes where and why.:)
 

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Discussion Starter · #8 ·
Hi...I think what's required is for you to take the time to follow the leads as they go through the amp starting either at the power end or input end. It's a relatively simple design and easy to do. It gets easier with practice. I get amps in that have no schematics available and guess what? They all share similar layouts.
It's best for you to discover what corresponds with the schematic as it will give you a clear idea of what goes where and why.:)
OK I guess the pots are also a good start point - I will get on it when I find the time - so much to do so little time- especially since I retired :)
Thanks
 

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Discussion Starter · #10 · (Edited)
OK So here is an additional question.

I have replaced all the tubes but that has not fixed one issue which is a hissing sound that starts say ten minutes after switching on. So I am thinking a bad capacitor - am I right?

For an amp this old is it recommended that all capacitors be replaced ? If not which ones should be replaced? Can the capacitors be replaced with 'better' options w.r.t. type?

Attaching the schematic for reference.
I made a list as best as I could read of the capacitors
C1 25/25
C3 0.001
C19 0.001
C20 0.001
C24 0.01
C25 0.01
C7 0.022
C11 0.022
C23 0.022
C2 0.047
C27 0.047
C5 0.1
C9 0.1
C12 0.1
C14 0.1
C15 0.1
C8 25/25
C28 ??/250
C6 10uF 450v
C18 10/40v
C26 125uF 16v
C13 220PF
C10 25/25
C17 25/25
C4 270Pf
C16 360pF
C21 40/450
C22 40/450
C30 40/450
C31 40/450
C29 64/184??

Can someone please fill in the gaps (??)

Thanks
 

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Discussion Starter · #13 ·
Next question :)

Attaching audio file a recording of the amp's hum from the speaker with master volume at zero.
Only appears when in 'operate' not 'stand by'
Does not increase with master nor preamp volume

Not a problem when playing loud but at low volume starts to intrude and annoy

Sounds like 120 hz not 60hz .

What causes this and it this cause for concern?

Uh oh - can't upload mp3 :(


Thanks
 

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if you pull the last preamp tube (phase inverter) does it kill the noise. if it does you can at least isolate the noise to the preamp. try pulling preamp tubes and see if removing any of the preamp tubes kills the noise. if it does you can narrow down your search to that particular stage.
 

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Discussion Starter · #16 ·
if you pull the last preamp tube (phase inverter) does it kill the noise. if it does you can at least isolate the noise to the preamp. try pulling preamp tubes and see if removing any of the preamp tubes kills the noise. if it does you can narrow down your search to that particular stage.
if you pull the last preamp tube (phase inverter) does it kill the noise. if it does you can at least isolate the noise to the preamp. try pulling preamp tubes and see if removing any of the preamp tubes kills the noise. if it does you can narrow down your search to that particular stage.
Hello - thanks for responding
OK I pulled each of the tubes v1 to v6 but the hum remains - so that points to the filter caps right?
 

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