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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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6th Oct 2019, 3:13 pm | #21 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Fender amp copy
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6th Oct 2019, 7:28 pm | #22 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Fender amp copy
Quote:
Colin The so-called Ampman. |
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6th Oct 2019, 7:42 pm | #23 | ||
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
Posts: 2,614
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Re: Fender amp copy
Quote:
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Richard Index: recursive loop: see recursive loop |
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6th Oct 2019, 7:59 pm | #24 |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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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. |
6th Oct 2019, 9:00 pm | #25 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Fender amp copy
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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8th Oct 2019, 12:19 am | #26 |
Heptode
Join Date: May 2005
Location: Adelaide, South Australia
Posts: 541
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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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8th Oct 2019, 2:30 pm | #28 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Fender amp copy
Quote:
Colin. |
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8th Oct 2019, 2:41 pm | #29 |
Heptode
Join Date: May 2005
Location: Adelaide, South Australia
Posts: 541
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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 |
8th Oct 2019, 3:06 pm | #30 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Fender amp copy
Quote:
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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9th Oct 2019, 1:30 am | #31 |
Heptode
Join Date: May 2005
Location: Adelaide, South Australia
Posts: 541
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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. |
9th Oct 2019, 10:45 am | #32 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 632
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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. |
10th Oct 2019, 7:05 pm | #33 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,536
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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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10th Oct 2019, 8:56 pm | #34 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 632
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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 |
11th Oct 2019, 2:05 am | #35 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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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 Joe |
11th Oct 2019, 7:18 am | #36 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,670
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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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Curiosity hasn't killed this cat...so far. |
11th Oct 2019, 7:35 am | #37 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 632
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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. |
11th Oct 2019, 9:08 am | #38 |
Heptode
Join Date: Sep 2010
Location: Hythe, Hampshire, UK.
Posts: 632
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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 |
11th Oct 2019, 3:20 pm | #39 | |
Octode
Join Date: Jan 2012
Location: Wimbledon, London, UK.
Posts: 1,465
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Re: Fender amp copy
Quote:
Colin. |
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