GRAMPIAN 562A

[chocopower]

Hi!!


New amp coming next week.


Schematic attached.
Any comment in any mods for guitar use?

[Joel in Texas]

Hopefully, some people smarter than me will reply too.

But here's an idea to start with, if it hasn't already occurred to you. 

After checking that critical parts work (transformers etc.) my first step in modding would be to re-do everything preceding pin 7 on V2 of your Grampian (not your Grandpa though - leave him and his pin 7 alone) with something that looks like the input stage of the Vox AC15.

Look here:
https://el34world.com/charts/Schematics/files/Vox/Vox_ac15.pdf

You could do your whole EF86 input stage like everything from J1/J2 to C27 in that one. 

Once you're getting guitar signal through the input stage in a way that suits guitar, using something like the above, you can evaluate what you need next.  More gain? Different frequency response?  Etc.

Anyway, that's where I would start.  Should be a good platform, from what I can see.

Have fun.

[Tony Bones]

To start (after making sure the amp is good to go as-is) I would remove everything before the input tube and replace it with an input jack, a grid stopper (68K?), and 1M grid resistor to ground. That's it!

Well, that's almost it. You'd end up without a volume control. To fix that I'd wire the existing 500K volume control in before V2. That'll get you up and running. If you want to keep the tone controls working as designed then replace R9 with a 1M log pot for volume control. There's no reason to think the tone controls are ideal for guitar as-is, but there's no reason to think they suck either. 

EDIT: It'll be interesting to find out how that PI works with output tube overdrive. The self-balancing feature (R13 & R14) might get wonky. It would be safe to connect the grid of V2b directly to ground, even without removing R13 & R14.

[chocopower]

Hi,


thanks for your coments!


My idea is test the amp as is, and them, as you said, rewire all before the ef86 with standard input arragament.


them, as Tony said, change the R9   1M grid leak resistor for a 1M pot.


That should be a good start point to tune the circuit.


One thing that seems odd to me, is that in the schemo they claim you can use 6L6 or 6V6 without any other coment about change the speaker load to acomodate the primary impedance load for each tube.
Amp is still traveling, so no test done, but i´m assuming, the primary load should be something like 6k6, who is in the upper range for 6L6 and in the lower range for 6V6.
with the 310v plate voltage shown in the schematic, i think will be ok for both 6L6 and 6V6....


worth a try....

[chocopower]

One more question...


A  EZ81 isn´t a bit in a hurry feeding 2 6L6G?  Voltage are really low in this amp, so maybe is ok....

[PRR]

I think this is a 6V6 amplifier. You always can put 6L6 in a 6V6 amp (if the heater winding allows), no change, get the *same* power output (15W). Normally 6V6 was a half-dollar cheaper, so why would you do that?? But some times and places, 6L6 was more available than 6V6, and it was good to have the choice.

[chocopower]

Thanks PRR.


when amp arrives ill check the O.T. to know the turns ratio.

[kagliostro]

Nice Score

Franco

[chocopower]

Hi.
Amp alive and working.
At this moment i just change the input with 1M leak resistor and 10k series grid resistor.
I use one of the 500k pot as volume before P.I.


Some questions.


1. Leak resistor for power tubes are 680k. Its over specs even for cathode bias (500k). Will be better upgrade them to 220k?


2.And in conflict with schematic, both are conected after the 4k7 grid resistor, forming a voltage divider.
Is this a factory mistake or maybe they did it in purpose to cut down a bit of signal?

[chocopower]

 


I just saw that r13-r14  are in parallel with r15-r16....


Rg1 is that way, "just" 400K

[66Strat]

They're not connected parallel. Resistors R13 and R14 connect to the grid of triode V2B. This is a self balancing see saw phase inverter.

On second look, there is a parallel connection to ground. R13 and R14 are connected in series with R16 for a combined resistance of 2.68 M which is parallel connected to ground with R15, for a grid leak resistance of 543K + 4.7 K (547.7 K) for V3. There is a mirror image grid connection for V4.

[DummyLoad]

get rid of that input transformer - add a 7 pin or 9 pin socket. socket for a 7 pin or another 9 pin tube EF86 to paraphase won't be enough gain to push over the top, especially with single coil PUs. have had something similar on the BB and was not pleased with the results.

remove input transformer and summing network leave 500K pot. insert 1 gainstage that feeds 500K pot. so - triode (vol) pentode (tone) paraphase (output) - trim NFB network - e.g. try 56k - 100K for FB resistor. fiddle with 330K G2 resistor for EF86 - go higher for more gain if desired: 560K-1M would a good starting point.

see attached. switched input jace at 33K/1M input node. no room so it is assumed.   

--pete

[PRR]

> 680k. Its over specs even for cathode bias (500k).

Yes. That mess of grid resistors is a mental maze. The 1Megs go to a small grid which is about infinite impedance so doesn't do the power grids much good.

On analysis I see that if *both* power tubes have grid current, each one "sees" 639k. If only one has grid current it "sees" 516k (and you can verify this in-circuit, power-off, with an ohmmeter). Since most tubes do not have big grid current, Grampian may have figured they were safe.

The 47k driver plate resistors would allow smaller grid resistors. Also self-bias 6V6/6L6 are not that hard to drive. I don't think it needs to be 220k. 470k seems fine. (Tack 1.5Meg across the 680k.)

Considering the age of that amp I assume you have checked all the resistors are near their marked values. If any doubt, fresh new resistors would be a best-bet.

[chocopower]

Considering the age of that amp I assume you have checked all the resistors are near their marked values. If any doubt, fresh new resistors would be a best-bet.

Just cheching now.... one of the 680k measure 800k....


PRR, sometimes, you should be tired to be right, isnt?


 

[chocopower]

Ok.. those 680k resistors upgraded to 470k.
A 22k in the power rail who raised to 27k, changed too.


Now. I made the turns ratio and impedance calculations for the O.T. 


It,s  8.5k.


I was missing a bit of high end, and made a test using a 8 ohm speaker into the "16 ohm" output.
Amp came to live.
For a couple of 6L6, 4.2k seems a better choice.

[chocopower]

Hi.


Amp is working quite well and i'm in the final procedures tweaking the tone controls and NFB.


Just one question. There are two 200 ohm resistors before rectifier. I've seen current limiter resistors for tube rectifiers before, but use to be a lot smaller (less than 50ohms).
I measured ac voltage before and after those resistors and they are dropping 25 Vac.


Could be that those resistors are there to protect the rectifier when amp is using 6L6 tubes?
If that's the reason, maybe could be removed if i just use 6V6 tubes?


Thanks in advance!

[kagliostro]

Valve Rectifiers
Ordinary valve rectifiers contain two diodes which share the same cathode (and heater), in one bottle. Valve rectifier data sheets usually state the maximum RMS transformer voltage that the valve can withstand in an ordinary two-phase rectifier circuit, rather than quoting limits in peak values like modern data sheets. The GZ34 data sheet quotes 550-0-550Vrms (although personally I wouldn't trust a modern production bottle to handle this). The data sheet will also quote the maximum average DC current that the valve can handle. For the GZ34 this is 250mA for a capacitor-input rectifier with transformer voltages up to 450-0-450V, but the limit is reduced for higher transformer voltages. The limit is higher for choke-input circuits, but guitar amps don't use those, so you don't have to worry about it. Most valve rectifiers also need their own, dedicated heater supply. The EZ81 is a notable exception.

In addition to the maximum AC voltage and DC current ratings, valve rectifiers have two other ratings that must be observed: maximum allowable reservoir capacitance, and minimum current-limiting resistance. These two limits are interelated and serve to keep the peak ripple current below a certain (unstated) level. The bigger the reservoir capacitance, the more limiting resistance you need. The GZ34 data sheet quotes a maximum capacitance of 60uF, although you can, in theory, exceed this if you increase the limiting resistance proportionately. However, this incurs extra voltage loss and wasted heat which is why the manufacturer assumes no one would want to do it.
The total limiting resistance (per anode) in the actual circuit is the combnation of transformer resistance, plus any resistance we add ourselves:

Rlim = Rsec + Rpri × (Vsec/Vpri)^2 + any extra resistance



Where:
Rpri is the DC resistance of the transformer's primary winding;
Rsec is the DC resistance of one half of the transformer's secondary winding, i.e measured from one end to centre tap;
Vpri is the primary (i.e. mains) voltage;
Vsec is one half of the secondary voltage, i.e. measured from one end to centre tap.

The data sheet will present table or graphs showing the minimum limiting resistance needed for a given application. If the transformer alone doesn't have enough resistance to meet this requirement then you need to make up the deficit by adding resistors in series with each anode. These resistors need to have a power rating that comfortably exceeds:

P = (1.1 × Idc)^2 × R

Alternatively, you could use one resistor (with twice the power rating) in series with the cathode.

Valve rectifiers have realtviely high internal resistance. This causes a significant loss of voltage which increases with load current, leading to power supply voltage 'sag' during loud passages. The data sheet will usually contain various graphs and tables of recommended operating conditions, some showing how much the voltage will sag with different load currents, so it should be fairly easy to extrapolate this information into your own design. As a rough rule of thumb, at full load a valve rectifier will produce a DC voltage that is between 1 and 1.2 times the advertised transformer voltage.

Vaccum rectifiers must not be 'hot switched', i.e. they must not be preheated with a standby switch before applying voltage to the reservoir capacitor, as this often causes flashover.

Source: http://www.valvewizard.co.uk/bridge.html

Franco

[DummyLoad]

those 200R on the anodes are the reason that PS can work with 50uF reservoir caps - i'd leave them be.


--pete

[chocopower]

Thanks.


Just learning

[chocopower]

Hi,
Amp had been working without issues and last week it blows the rectifier and the 250ma fuses.


I replaced the fuses, check the amp without tubes and bulb current limiter and everything seems ok.


Install a new EZ81, and it work for several hours and did the same again....


Could be the old filter caps that are killing the rectifier?


I know... i should change them when i worked in the amp, but i tested them, and were ok...