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Old 6th Oct 2019, 3:13 pm   #21
Paul JD
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Default Re: Fender amp copy

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I didn't think Fender amps went up to 11?
Many of them go to 12! The tweed covered models from the 1950's all have controls that go up to 12.
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Old 6th Oct 2019, 7:28 pm   #22
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Default Re: Fender amp copy

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Originally Posted by mole42uk View Post
I didn't think Fender amps went up to 11?
Many of them go to 12! The tweed covered models from the 1950's all have controls that go up to 12.
They did indeed! I stand corrected (actually it's more appropriate that I slink away in shame at not looking it up)...

Colin The so-called Ampman.
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Old 6th Oct 2019, 7:42 pm   #23
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Default Re: Fender amp copy

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Originally Posted by mole42uk View Post
I didn't think Fender amps went up to 11?
Many of them go to 12! The tweed covered models from the 1950's all have controls that go up to 12.
They did indeed! I stand corrected (actually it's more appropriate that I slink away in shame at not looking it up)...

Colin The so-called Ampman.
I'm rather afraid that my comment was an attempt at a humorous Spinal Tap reference. I'm only sorry that I seem to be on my own special planet....
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Old 6th Oct 2019, 7:59 pm   #24
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Default Re: Fender amp copy

I understood that reference, Richard, so my shame at not doing the research isn't your fault. Long may Christopher Haden-Guest live (I envy him being married to Jamie Lee Curtis, though).

Colin.
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Old 6th Oct 2019, 9:00 pm   #25
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Default Re: Fender amp copy

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I'm rather afraid that my comment was an attempt at a humorous Spinal Tap reference. I'm only sorry that I seem to be on my own special planet....
No, I got the spinal tap reference (great film!). I once built an amp for a customer who wanted it to go to 12 so he could claim to be one louder than spinal tap - when I told him some Fenders already go to 12 he was most disappointed and asked me to build it so it would go to 13!
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Old 8th Oct 2019, 12:19 am   #26
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Default Re: Fender amp copy

I did see Spinal Tap many years ago, the only parts I remember are the guitar that had so much sustain you could go away and make a cup of tea and come back to continue playing and the famous volume control to 11 that everyone seems to recall.
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Old 8th Oct 2019, 12:30 am   #27
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Default Re: Fender amp copy

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... the only parts I remember are the guitar that had so much sustain you could go away and make a cup of tea and come back to continue playing ...
See here
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Old 8th Oct 2019, 2:30 pm   #28
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Default Re: Fender amp copy

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I did see Spinal Tap many years ago, the only parts I remember are the guitar that had so much sustain you could go away and make a cup of tea and come back to continue playing and the famous volume control to 11 that everyone seems to recall.
IIRC, the guitar with the sustain was a Gibson Les Paul. I think that it may have been the same guitar that "wasn't for playing, just for looking at".
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Old 8th Oct 2019, 2:41 pm   #29
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Default Re: Fender amp copy

He didn't go away and make a cup of tea -seriously though it is impressive.

I delivered the amp only to get a call a few hours later - HT fuse blew. Back on the workbench I found a partial short to ground on the tremelo depth pot, the circuit I used was a modified version which varies the bias voltage to produce tremelo - internet opinion seems to be that it is more reliable and capable of more depth than the neon/ldr setup, the partial short to ground shunted the bias voltage to ground as the tremelo depth control was turned up, both input channels equally affected. Turning the depth control to 4 or 5 reduced the bias from -36V to around -18V, enough to blow the HT fuse.

My son has my Les Paul Custom that I purchased second hand when I first started work back in 1970 and wanted to be a guitar god, cost me 6 weeks pay on lay-by, sadly I had no talent and it ended up shoved under the bed.

Last edited by retailer; 8th Oct 2019 at 2:46 pm. Reason: need to add extra
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Old 8th Oct 2019, 3:06 pm   #30
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Default Re: Fender amp copy

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He didn't go away and make a cup of tea -seriously though it is impressive.

I delivered the amp only to get a call a few hours later - HT fuse blew. Back on the workbench I found a partial short to ground on the tremelo depth pot, the circuit I used was a modified version which varies the bias voltage to produce tremelo - internet opinion seems to be that it is more reliable and capable of more depth than the neon/ldr setup, the partial short to ground shunted the bias voltage to ground as the tremelo depth control was turned up, both input channels equally affected. Turning the depth control to 4 or 5 reduced the bias from -36V to around -18V, enough to blow the HT fuse.

My son has my Les Paul Custom that I purchased second hand when I first started work back in 1970 and wanted to be a guitar god, cost me 6 weeks pay on lay-by, sadly I had no talent and it ended up shoved under the bed.
I don't much care for the bias-voltage tremolo. It affects both channels as you say and this can be a nuisance if you have a microphone going into the other channel. Tremolo'd voice sounds pretty horrible.
The other objection is that the optimum grid bias on the output valves is being compromised. I tried an experimental hook-up some years ago and I could see the change in current through the output valves varying by noting the red glow fluctuating. I soon re-jigged the circuit. OK, the neon/ldr circuit has its detractors, but I think Fender stayed with it for good reasons.
Colin.
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Old 9th Oct 2019, 1:30 am   #31
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Default Re: Fender amp copy

I did give the tremelo some thought, the neon/ldr simply shunts the signal to ground, simple.

The bias voltage tremelo at first glance looks to compromise the bias voltage however if you look at the circuit it is in essence a LF oscillator, capacitor coupled to the output valve grids - the guitar signal does exactly the same thing, I may be very wrong here but I can't see any difference apart from the frequency of the two signals, the guitar signal and tremelo oscillator both vary the output valve grid voltage. The upside to the neon/ldr idea is that only the vibrato/reverb channel is affected - like you say if the amp is shared between two band members then there is a problem with the bias voltage method. There were quite a few Fender amps that used the bias voltage tremelo, perhaps this is why Fender moved away from this idea, I have heard it was quite common in the 60's for an amp to be shared between two band members.
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Old 9th Oct 2019, 10:45 am   #32
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Default Re: Fender amp copy

I recently repaired a Fender Reverb Pro and your build is a good Fender copy.

The Fender I had in the workshop had 2 speakers with those and all the transformer ironmongery I could barely lift it onto the bench!

For interest I have attached the generic circuit for a few variants of the 70W RMS amp.
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File Type: pdf Fender 70W RMS Amp.pdf (173.2 KB, 30 views)
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Old 10th Oct 2019, 7:05 pm   #33
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Default Re: Fender amp copy

70W from a pair of 6L6 (even the GC variety) is caning them a bit isn't it?
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Old 10th Oct 2019, 8:56 pm   #34
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Default Re: Fender amp copy

Not for the 6L6GC specified in the design as these are rated at a plate dissipation of 30W so a pair in class AB push pull is well within the rated power.

It's interesting that the original 6L6 was rated at 11W but subsequent design improvements managed to push this to what is now seen in the 6L6GC
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Old 11th Oct 2019, 2:05 am   #35
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Default Re: Fender amp copy

I think Herald has it!!. I have repaired many many Fender amps, and even a twin reverb with 4 6L6's MIGHT make 70 watts. RMS, English style not American style RMS. Did you actually do an E^2/R measurement?.

with respect

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Old 11th Oct 2019, 7:18 am   #36
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Default Re: Fender amp copy

"Not for the 6L6GC specified in the design as these are rated at a plate dissipation of 30W so a pair in class AB push pull is well within the rated power." AFAIK you don't double power OP, EG it's not 2 x 30w as only one valve is working in the class B part of cls AB PP at a time, therefore two 6L6's biased at 30w each will give you 30w OP.

Re tremelo circuits I'm working on a WEM amp, see attached schematic. The amp has separate mic IP's which give a clean hum free OP, but using the tremelo IP's results in quite bad hum, this is refardless of the tremelo being on/off. I've been over the wiring/layout and checked components but am convinced the hum is down to the design. What's your circuit like as regards noise? Are tremelo circuits noisey on the whole?

Andy.
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Old 11th Oct 2019, 7:35 am   #37
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Default Re: Fender amp copy

I reference http://www.r-type.org/articles/art-123c.htm

Which states:

Class AB relates to the case where the grid bias point lies between those which apply for Class A and Class B operation. Class AB working can be used with two valves in an AF push-pull output circuit, and it enables a higher power output to be achieved than would be given by the same valves in Class A.

A Class AB amplifier functions in the same manner as a Class A amplifier for small signal inputs, because the input waveform is still applied to a linear part of the IaVg characteristic. As input signal amplitude increases, the negative parts of the grid waveform become applied to the more non-linear section of the IaVg characteristic, and the distortion offered by each valve increases. However, the ability to cancel out even harmonics which is given by a Class A amplifier tends to apply here also, and this distortion is reduced. At yet higher input signal amplitudes each valve may be cut off during negative input peaks, whereupon one valve only passes anode current during these periods.

Since two valves in Class AB offer a greater power than the same valves in Class A, Class AB operation is attractive for applications where an increased distortion at high output powers is acceptable. With reasonably careful design, Class AB operation can also be used for the output stages of domestic high quality amplifiers.

Class AB stages may be divided into Class AB1 (where no grid current flows) and Class AB2 (where grid current flows during part of the positive input half-cycle). It is normal practice for the Class AB2 grid bias point to be closer to cut-off than occurs in Class AB1, and Class AB2 gives a greater power output. One sometimes encounters the statement that Class AB2 is intermediate between Class AB1 and Class B. For domestic amplifiers, Class AB1 is preferred because the output stage can be preceded by a simple voltage amplifier circuit, instead of by a power amplifier as would be required with Class AB2. Also, the distortion introduced by the output stage will be lower with Class AB1.

Unlike a Class A push-pull output stage, a Class AB push-pull stage draws an increased HT current as input signal amplitude increases. The increase in HT current is not as marked as with Class B operation, and does not preclude the use of cathode bias for Class AB1 working. An external source of grid is, however, required for Class AB2.
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Old 11th Oct 2019, 9:08 am   #38
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Default Re: Fender amp copy

While I agree you cannot exceed the total power dissipation of the valve, it is perfectly possible to work it harder for a shorter duty cycle. If we look at the original RCA 6L6 data it shows a total power dissipation of 24W. If we look at the data for Class A1 (fixed bias) we see a max dissipation at peak signal of 375V x 73mA ~ 27W and with zero signal the dissipation is ~ 22W delivering 11.5W of signal output.

Now look at the data for class AB2 push pull we see a max dissipation at peak signal of 400V x 125mA (for one valve) = 50W and with zero signal the dissipation 21.6W (for one valve) with a pair delivering 60W signal output. The valve is effectively over rated by a factor of 2.08.

If you do the efficiency calculation then Class A1 is about 42% and Class AB2 is about 60%. If all things are equal applying this to a 6L6GC capable of 30W dissipation the maximum signal power output would be ~75W
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File Type: pdf 6l6.pdf (255.2 KB, 10 views)
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Old 11th Oct 2019, 3:20 pm   #39
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Default Re: Fender amp copy

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70W from a pair of 6L6 (even the GC variety) is caning them a bit isn't it?
Fender did indeed cane output valves by giving them a higher anode voltage than was specified by the manufacturers. Two 6V6s in a Fender Deluxe Reverb are given 415 VDC on each, which is more than the recommended 315 VDC for the Tung-Sol 6V6GTA (absolute maximum). The result of this was that some 6V6s died quite quickly. Currently, the only 6V6 that will work for a long time in a Deluxe Reverb are the Electro-Harmonix branded 6V6GT, which is made in Russia, presumably by Reflektor.

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