troubleshooting M40 Echolette with loud hum

[bea]

I am still using the old value of 200 Ohms, but i intend to use the M40 mainly as a PA and as a small bass amp. With the bass i try to avoid to overdrive the power stage - but who knows, i managed to overheat the screen resistor of a Dynacord Bassking 1. That resistor still has its value of 820 Ohms, but i did replace it. In that case by 1.2 kOhms - which Dynacord also did somewhat later in their amps.

So dependent on Your application You should use 220 Ohms or larger. I am a bit unsure but actually the screen voltage should be slightly below the anode voltage, not slightly larger (which is a consequence of the ultralinear setup). You can estimate by Ohm's law how large it would have be to accomplish this. So 330 kOhms should be a lot safer, even a larger value if You intend to overdrive the stage massively.

And seriously - a resistor costs a few cents. Changing a damaged but more or less working part to an intact one will cost You almost nothing it can save a lot of money. Please keep in mind that these overheated resistors fail a lot easier than new ones, especially that one which was really short of total failure.

[bea]

If you intend to use the amp ,mainly for guitarwork it is adviseable to reset the topology for the entry triodes. Just look up a favorite design of yours and copy..

Background: the configuration of the entry triodes is a bit "surprising", even if they a re used with microphones.

Please let me quote a contribution on that stage i have recently written in a german forum. To summarize: the ECC81 fits a lot better into the configuration of the input triodes than the ECC83 does. The reasons are given below. Swapping ECC81s into the input stages gives a much better tone with guitar and bass - less distortion, larger input swing and larger amplification.
This evening i will hopefully be able to check this with a microphone as well.

Hier mal ein paar Daten, entnommen dem Plan der Echolette M70, der glücklicherweise mit Spannungswerten versehen ist. Die M70 ist mit ECC808 bestückt. Der Vorverstärker unterscheidet sich aber sonst kaum (in der Master-Klangregelung!) von dem der M40. Das gilt auch für die Siebkette und damit mit zumindest hinreichender Genauigkeit für die Versorgungsspannungen.

Hier also Daten der Stufe: Rk=2.2 k, Ra=22 k, U=160 V, Ua= 122 V.

Damit ergibt sich ein Ruhestrom von ca 0.7 mV (es gibt 4 Systeme).

Mit diesen Daten kann man die Verstärkung der Stufe auf knapp 24-fach abschätzen und den Innenwiderstand auf 18 k.

Die Aussteuerbarkeit des Eingangs liegt mit Ach und Krach bei +- 1.2 V bei starker Unsymmetrie --- zu wenig für Tonabnehmer mit "normalem bis hohem Output". Diese 1.2 V mal 24 führt auf einen Spannungshub von +- 29, also wohl ausreichend für den maximalen Spannungshub.

Mit der ECC81 ergibt sich als Verstärkung ca. 38, der Innenwiderstand sollte unter 10 k liegen. Als Ruhestrom sollten sich ca. 1.1 mA ergeben.
Die Aussteuerbarkeit des Eingangs beträgt -2.2 V/+1.7 V. Sie ist also etwas größer als mit der ECC83, aber die Verhältnisse sind nicht wesentlich günstiger.
Wegen des höheren Ruhestroms sollte die Anodenspannung etwas geringer sein und einen größeren Ausgangsspannungshub ermöglichen.

[notea]

this is a really interesting configuration, i will try the ECC81, you're right about the resistors i'm gonna order new ones and replace them, to be sure (cause i'm a beginner) i should stick to the 220 ohms for pin 9 and eventually go higher for the 2k of pins2 to about 3k3 ohms? and for the wattage, should i go for 5w metal oxide for example?
I've sent an email to Mr Welter to see if he could supply me a schematic to connect the PT but i don't think he can, the easiest way for me would be to find a NOS one.

[bea]

Please dig into the literature and try to understand the function of the resistors. he material of this page will be pretty helpful - it is concise but still written very clearly and suitable for beginners (i am also a beginner): How to design valve guitar amplifiers

The grid stoppers (2.2 kOhms) do not need to be increased. The screen resistors can of course be metal oxide, but they should not be chosen as a larger wattage - it is intended that the'll fail before the screen grids.

[notea]

thank you bea for the link, i bought the book seems pretty complete, i'll have some homeworks to do.I'll wait a bit before ordering the new PT, Mr Welter sent me a picture of the PT with the voltage labelled on it and the terminals position are quite different compared to the original, don't want to ruin a new PT, i'll wait a bit to see if i can find some interesting auctions on ebay

[notea]

Hi everybody,

i'm back on the Echolette, we identified few months ago that this loud hum was due to a bad PT. I found a non working unit on ebay hoping that the power transformer was not shorted. I swapped the PT, but nothing happened. So i try to replace the power switch and there it goes, it's alive!
The problem is the Echolette is not dead quiet, i still got a little hum that's increasing as i turn the volume pot.
The more annoying is a quite loud static noise that's also increasing as i turn the volume pot. There is four input and when i play with the volume of each channel there is some scratchy noises and don't know if it's normal but when i flip over the unit and play with the volume of each channel and touch the channel region as the same time with my wooden stick, it is very microphonic, and there is static noises also when i touch the gain pots (with knobs removed) with my fingers.
-could it be a ground issue, Do i have to replace all the caps in the preamp section ?
I've already replaced the leaky coupling caps that goes on the 12AX7's
- do somebody know what the 2 small pots near the output valves and one near the preamp are for?
i tried to tweak those 2 pots near the output valves with my meter pluged on pin2 (of one of the El84) to see if there is a bias pot between those two but nothing happened..

If someone has an idea, i'll be really grateful, it seems that i'm pretty close to a fully working unit

cheers Nick

[bea]

Hi Nick,I had to check

one of the pots adjusts the symmetry of the heating of the preamp tubes relative to ground (100-500 Ohms), contact area a relatively thick wire. Somewhere near the centre the hum should have a minimum. A 2nd pot adjusts the level of the tape recorder output in the echo socket on the front (Ausgang III) - some 20 kOhms. The third pot (1M) adjusts the level of the input to the stage after the tone stack.

I had to have a look into the schematics for the 2nd and 3rd trim pot.

Scratching may be some dirt in the pots of the amp. In one of my M40 i had to turn them left to right for several times to get them less scratchy.

Addition: with the exception of maybe the latest version of the M40 and its technically equivalent successors (the M70) the amp does have fixed bias but it does not have a possibility to adjust the bias. Quite interestingly it seems to run safely with pretty large loads - the two speakers the amp needed have an impedance of nominally 4.5 Ohms (each, i.e. total load roughly 2.5 Ohms) and can be run in parallel on the 6 Ohm outputs.

[notea]

Hi Bea, thanks for that i'll try to tweak this hum balance pot to see how it goes, i'll do a big cleaning of all the pots see if it's getting better,
this noise is really annoying, i might order a new set of tubes also.

On the back up unit i bought, there is 2 diodes on the first rectifier tube socket (see attached pics) not sure but it looks like a diode protection mod maybe to prevent the PT from a broken rectif tube,
am i right? Could it be worth it to install this mod on my working unit?
the 6 ohms resistance could be a good idea to smooth out the HT a bit before the filter caps

this the measuring on the EL84:

pin 2 = -12 VDC

pin 7 = 348

pin 9= 346

edit: this static noise i got may come from a DC or AC leakage but i'm not sure, i connected my guitar and plugged my meter on the strings and i can read an oscillation of -17 to +17 mv
is it normal? the grounding point near the power cord section seems to be ok, no rust deposit or some kind.

cheers Nick

[bea]

Mhmm, the diodes might be intended to fully replace the valve diodes (which possibly might not be needed at all. The 6.8 Ohms resistor might represent the internal resistance of the valve diodes and protecting the main transformer from to large initial current. It would be really helpful to draw the actual plan. Are the valve diodes (the 2 EZ81) connected at all? Are the silicon diodes ok? IMO there should be only one rectifier, not two in parallel.

Probably in the 70s or 80s the rectifier section seems to have been fully replaced. The electrolytics seem to be from that time. Is the 2.2 k resistor between the caps present?

BTW: the voltage readings are ok. In the M40, the EL84 are run beyond specs.

It is possible that You need to replace the two large electrolytic caps again. Possible source: Frag Jan zuerst --- Ask Jan First

[notea]

thanks Bea, ok it makes sense for the diodes of this defect unit, i'll not go further with this one cause i putted out the PT already but when i received it i was curious about power it on and see how it goes and it was funny to see that it was showing the same symptom; a huge amount of hum.. (If you're interested about this diode mod i can do a drawing for you if you want but i guess you know how this thing works)
now it 's good to know that i can take some spare parts out of it when i need it.

Happy to know also that my EL84's reading looks ok for you cause i was worry about overheating problem

As for the filtering caps the previous owner already changed it with 2 caps in series 150+150uf 450V but even if there are new you're probably right for replacing them, they might have become weak after all those tests.
Thanks a lot for your link there is a wide range of caps on this site!
I know it's wrong but i'll try to swap with the filter caps i got from the spare unit, one seems to be leaky but the two others seem fine apparently.. we'll see if there is any difference

Cheers Nick

[notea]

Hi everybody,

it's been a while now but i'm back on the M40 after a long break,
the amp dosen't work properly, there is no hum anymore but the sound is realy weak, distorded, the current noise is as loud as the guitar notes..
It sounds like it was a bad tube but all tubes are new exept the EZ81.
- I changed all the electrolytic caps by new ones, changed the plate resistors etc..
- I swapped the input din sockets with jack sockets and also the output, but i might did wrong (please see the attached pics)
i don't where to look at now, if somebody went through this problem and could help me on this i'll be grateful

- i didn't change the 1500pf & 2000 pf caps around the volume control
this are the measurements without the output tube plugged in:

Pin 9, Usg: 343
Pin 7, Ua: 341
Pin 2, Ug1: -12

cheers Nick

[bea]

Now You might understand why i left the DIN sockets and use adaptors...

The voltages in the power stage look ok. Quite probably the problem is elsewhere. Even when the EZ81 are nearly worn out there should be at least a "reasonable" output. Not ideal, but better than what You describe.

What about the other tubes? If You need reference voltages - there is a plan of the M70 (same amp but built with ECC808) which contains all voltages.

And then You need to enter a signal and look which stage fails (with an oscilloscope) - successively from stage to stage. either front to back or back to front.

[notea]

Hi bea, unfortunately i don't have oscill. i poke around with my sound test probe on each grid to inject a tone but it wasn't conclusive,
here are my new measurements with all tubes removed
i don't know if it's normal to have AC readings on pin2 of Rö3&4?

4x El 84=
Pin 2 : -12v
Pin 7 : 378
Pin 9 : 374

Rö1,2,5= 361V(pin1&6) and 1VAC (oscillating on pin2)
Rö3= 352v(pin1&6) and 16.8VAC (pin2)
Rö4= 311v(pin1&6) and 12.6VAC (pin2)

in the attached file this is sample of the sound i've got out of the hp
17042016.m4a

[bea]

No it is not normal. The grids should be at 0. Maybe the coupling capacitors are defective. 1 V AC on the grid of the inputs is also suspicious. Are You sure there is no contact between the OT and the speaker connectors (again, i find it really brave to replace the DIN connectors - there is really no need for that)

[notea]

yes i know but i thought it would be easier, more convenient for giging (when you don't have all your head like me and you forget your adaptator..)
I changed the 22 nf cap, one 1M resistor that was drop at 960k on Rö3, swap a 0.1uf cap with a nos one on Rö4 (don't have new ones for the moment)
and it is way better, more gain, the notes are more present but still there is this horrible current noise that when i touch the chassis it's almost gone..
i've got something wrong with my ground, i checked the output and it's ok nothing touch the chassis, i cleaned the ground spot near the HT socket that sometimes it's a bit corroded
but still got this noise.. i might look around those 0.1uf (C20,21,23) that connect to ground and order new ones a part from that i don't know where to look at..
cheers