OD trim voltage value

[heisthl]

Are you using a .047 mid cap now? If so, you can implement this - although right now you have doubled up on the relay contacts so you will have to remove the jumpers across the relay and move some associated components to get one side of the relay freed up. Then you can use almost the exact same method - common to mid pot (don't forget the 4m7 to ground) A .01 across the other 2 terminals with the normally closed terminal being fed by the .047 on the main board.

[Guitarman18]

talbany wrote:

The grid resistor on the preamp stages typically ranges from 0 to 68K, although very large values, such as 470K, are sometimes used in high-gain preamps to shape the frequency response and prevent "blocking" distortion in the preamp section under heavy overdrive conditions. The Miller capacitance of a typical 12AX7 is around 151pF, so the upper frequency response -3dB cutoff point of a stage using a 68K grid resistor is around 15.5kHz. (Lower the resistor higher the rolloff) The frequency response drops to around 2.2kHz if a 470k grid resistor is used. This "free" response rolloff can be used to tame the "buzziness" of high-gain preamp stages without having to add additional rolloff capacitors. Perhaps the most important grid resistor is the one that goes to the grid of the very first stage, right after the input jack. This resistor is the one that prevents oscillations and pickup of radio stations and other noise due to long or poorly-shielded cables. It is not usually a good idea to eliminate this resistor. Ideally, it should be soldered directly to the grid pins of the socket, with very short leads

From Aiken

If dat resista goes bad take it back as that should be covered all up unda neef da wawentee!!

Thanks for the info Tony. Knowing that the typical Fender value is 68K and HADs' are generally 22K (33K-#124), I was curious as to the significance of the value when setting up the ODS for use with S/C p/u's. Reading through Aiken's words I get the impression that in these type of amps, the value is not too critical on V1a, as long as there is something within ballpark. I suppose the best way to find out is to have a go at changing the values in my amp!

Cheers,

Paul.

PS. Apologies for the unintentional derail of the thread.

[bcook]

Actually, on the typical fender amp, plugging into input 1 puts the two 68k resistors in parallel.

[Guitarman18]

bcook,

Actually, on the typical fender amp, plugging into input 1 puts the two 68k resistors in parallel.

Oops Of course you are correct. What I should have said is...

'Knowing that the typical Fender value is different to HADs' (generally 22K- (33K-#124)), I was curious as to the significance of the value when setting up the ODS for use with S/C p/u's'.

Cheers,
Paul.

[bcook]

'Knowing that the typical Fender value is 34K and HADs' are generally 22K (33K-#124), I was curious as to the significance of the value when setting up the ODS for use with S/C p/u's'.

Cheers,
Paul.

In the first stage, it's really not going to have much of an effect on the tone unless the resistance is large or you like to blast the amp input with a boost pedal. There are better places to tweak.

[talbany]

Two input jacks – found on Fenders and Marshalls (the Marshalls in question have four jacks.)

Why is one high gain and the other low? And how does the whole thing work, what are these resistors doing?

Overall the two high and low gain jacks are a very good trick using switching jacks and as few parts as possible.

Jack #1, the high gain jack has a 1 meg (1,000,000 ohms) resistor wired across the jack from the hot tab to the ground tab, and the grounded lead of the resistors is bent back and soldered to the middle tab – the “switch.” The switch is grounded on jack #1.

These switching jacks have an extra “leaf” that is touching the hot, or “tip” connection of the jack – but only when nothing is plugged in – called a “normally closed” jack. This can be made to do all kinds of things.

Trick #1. In the case of the #1 jack, when you do not have your guitar plugged into the amp (either jack) the “switch” is closed – since it is grounded, the amp is turned off for low noise, and no chance of runaway.

The 1 meg resistor is the “grid load” resistor for the first tube stage of the amplifier. All tubes have to have a grid load resistance or they will not operate.

Instantly many of you are thinking, “what about the guitar pot, or the pickups, aren’t they a grid load?” Yes, they would be – but – if the guitar had active electronics, or had a capacitor in series with the output signal from the pickups, there may be no direct resistance from the guitar. It is better to be safe and have the resistor there.

Ok, more on the input jacks. The second jack has it’s switch soldered to the first jacks hot lead. Each jack has a 68k (usually) resistor from its hot lead, and they join together to make one lead to the tube stage.

These “series resistors” are sort of input buffers – to keep the amp from getting high level spikes from the instruments. Not totally necessary for the vintage guitars these amps were made for, but a real good idea for modern guitars with high output pickups, and in the old days, accordions!

Here is trick #2. Jack #2 has it’s switch soldered to Jack #1’s hot connection, and a 68k resistor on it’s hot lead – when the switch is closed – nothing plugged into jack #2, it’s 68k resistor is placed directly in parallel with Jack #1’s 68k resistor, for half the value, 34k. Less series resistance.


Trick #3. If you plug into jack #2 only, check out what happens. We already learned that if nothing is plugged into jack #1 the hot is shorted to ground via the switch – so now Jack #1 represents a direct ground connection.

The signal from jack #2 would pass through the 68k resistor, and at that point see the 68k resistor from Jack #1 with its other end grounded!! So the signal has 68k in series (more resistance, a lower signal level gets to the amp) and a 68k grid load resistor – much lower resistance than the 1 meg. A lower value of resistance to ground reduces the signal to the tube (again.) Check out the schematics to see what is going on.

Tony VVT

[angelodp]

Can you roll into the Tweed Deluxe topology. Why does it sound so damn good.

[talbany]

Can you roll into the Tweed Deluxe topology. Why does it sound so damn good.

Interesting Question.. In my opinion much of the magic in the 5E3's circuit is not what's in it but what's not in it.....Very minimal in the way of tone shaping.. Out of one tube into another with very few components.. My theory here is when Fender came up with these amps they wanted cheap bare bones.. Lets build an amp that just amplify..No reverb, tremolo etc and do it with as few parts as possible...Most likely not realizing that the simpler the design the better the sound, feel, expression..This is still true today..a Zillion amps later.. Ken Fisher felt the same way.. Build a very simple design and load it with some of the best tubes ever made... Can't go wrong..+ That's my opinion..


Tony VVT

[jelle]

Can you roll into the Tweed Deluxe topology. Why does it sound so damn good.

If I may answer that one as well....



Simple.

See-Saw Phase inverter distortion.

[talbany]

See-Saw Phase inverter distortion

Cathodyne Phase inverter like the Princeton is a cheap way of phase inversion...1/2 tube..Uses the 180 degree phase shift between the plate and cathode...They were less balanced than the long tail creating more asymetrical clipping...Aysemetrical clipping sounds thicker to my ears but has a nasty unpleasant breakup..When you crank the amp into output distortion get the 6V's to sing you don't notice this as much...6V's cathode biased pushed to clipping =smooth sing sustain...To me much better than EL-84's... Jelle's right big part of the charm!!
http://www.freewebs.co.uk/valvewizard/cathodyne.html



Tony VVT

[Tavda3172]

Hi Guys,
I believe the 5C3 and 5D3 use a see saw phase inverter, relating to jelle's comment, and the 5E3 uses the cathodyne (concertina) relating to Tony's. Both produce asymmetrical clipping but in the case of the see saw, it has the distortion of an extra triode on one of the drive lines. Each has a different shade of tweed !
-Kevin

[talbany]

Hi Guys,
I believe the 5C3 and 5D3 use a see saw phase inverter, relating to jelle's comment, and the 5E3 uses the cathodyne (concertina) relating to Tony's. Both produce asymmetrical clipping but in the case of the see saw, it has the distortion of an extra triode on one of the drive lines. Each has a different shade of tweed !
-Kevin

We did some work not long ago on one of Bill Kirtchen's original 5C3's Talk about beat up..Whew!!It had been converted to 12ax's..I liked the breakup qualities of the see saw a bit better..That amp definitely had some mojo going on...I think Bill said he used that amp on an upcoming Elvis Costello release..He also said he bought the amp new..One of his firsts...Amp covered in nostalgia..

T

[BobW]

There are some very simple circuits found in guitar amplifier that we take for granted. But at the same time, do not understand. And there are other things thought to be “magic” , again, because they are not understood.

Your statement is invalid, and sounds like something either Gerald Weber or Dan Torres would have said. (a 40% baloney factor). It obviously does not apply to most here on this forum who know and understand how to read a basic schematic. I do appreciate your inputs when they are based on facts and less on opinions. The audience here is composed of electrical engineers, amp techs, commercial amp manufacturers, newbies and anything and everything in between. However, one thing we do all have in common is to build and play a great sounding amp.

[heisthl]

T And there are other things thought to be “magic” , again, because they are not understood.

Your statement is invalid, and sounds like something either Gerald Weber or Dan Torres would have said. (a 40% baloney factor). It obviously does not apply to most here on this forum who know and understand how to read a basic schematic. I do appreciate your inputs when they are based on facts and less on opinions. The audience here is composed of electrical engineers, amp techs, commercial amp manufacturers, newbies and anything and everything in between. However, one thing we do all have in common is to build and play a great sounding amp.

Not true - I removed a TRIAD transformer from an original Fender "The Twin" amp (50's) and carefully scraped all the resin of the windings and ground it into a very fine powder that I carefully sprinkle over the components in each build during final test. I can't explain it but the "magic dust" makes all the difference.

[BobW]

Henry, you know you missed your calling.