Replacing old resistors in tube amps

slimecity

Super Member
Hi - I have a NZ built Fountain 10-10 10 watt ECL86 integrated amplifier. I love it

The issue is that there are 2 x ECL86 per side - in this case Telefunken - and there is a hum that follows the tubes when you swap sides.

in order to rule out component issue possibly contributing to this - I want to ensure the resistors are OK. The former owner replaced 80% of old resistors - with carbon types. But please see some of the old ones in the centre of the photo which he didnt replace - shoud these types also be replaced? There are about 10-12 of these left in the amp.

I have also recapped this amp and there are only a couple left to replace with good Panasonic types.

cheers!

IMG_4012.jpg
 
Cheers for that. The problem is that this amp was made in NZ in the bad old days of closed borders - so these resistors could be of any quality. The trafos seem to be good quality as the amp sounds great.

I wasnt sure what type of resistor these are - but now you have told me, please see this note on this page. It makes me think i do need to replace them. On firing up, I can see filaments on one side of the ECL86's glow bright red for a few seconds before calming down again.....

Just a Note: When these have not been restored the output valves (ECL86 x 4 ) will often glow red, some are quick to blame the valves! but it is most likely the carbon composite resistors that have gone way out of specification.


https://solidstatevalvetube.weebly.com/fountain-gemsonic.html

Any views on this? cheers
 
Swapping valves can be a quick indicator that a tube or tubes have degraded in some aspect, or that imbalance (especially in PP output stage) has become a problem. Imbalance in a PP output stage can be checked by measurement of cathode currents in idle state. Degraded heater-cathode resistance, or grid gassy conduction in valves is most easily checked by tube rolling. The hassle with ECL86 is that either of the two valves within could be at fault.

Valves are by far the first parts to point a finger at, then checking passive parts like resistor tolerance and capacitor leakage.

If the amp is then all working correctly, by measurement of voltages around the amp, and by passing signal frequency sweep through the amp, and by confirming no onset of instability via square wave test, then further assessment could be warranted to reduce hum and noise floor through a wide variety of checks including how grounding and wire layout was done.
 
Valves are by far the first parts to point a finger at, then checking passive parts like resistor tolerance and capacitor leakage.
Awesome cheers. I will be rolling to PCL86 as the ECLs are now getting too rare and expensive. This will of course involve a heater voltage/current mod.....
 
if they measure in tolerance, the resistor is almost certainly OK. CC can get noisy, but its usually related to moisture, which is also how they go off value. I've never had a noisy CC that was not off value by a considerable amount.

Often you can measure them in circuit and get a good reading. Anything across them will make it read low, if they read high its a bad resistor.
 
Cheers for that. The problem is that this amp was made in NZ in the bad old days of closed borders - so these resistors could be of any quality. The trafos seem to be good quality as the amp sounds great.

I wasnt sure what type of resistor these are - but now you have told me, please see this note on this page. It makes me think i do need to replace them. On firing up, I can see filaments on one side of the ECL86's glow bright red for a few seconds before calming down again.....

Just a Note: When these have not been restored the output valves (ECL86 x 4 ) will often glow red, some are quick to blame the valves! but it is most likely the carbon composite resistors that have gone way out of specification.


https://solidstatevalvetube.weebly.com/fountain-gemsonic.html

Any views on this? cheers

Unlikely the momentary brightness you see is related to the resistors.

The reference "Just a Note" is to red-plating of the anode/plate, not filament flash.
 
Last edited:
Certain British mullards momentarily flash upon startup, that is normal and not cause for concern.

In fact it indicates you have a very good quality tube ,most likely Mullard.

It is also unlikely those resistors have anything to do with the filament circuit . also they appear to be gold tolerance which is better than average , physically they could be 1 watt .


Anyway, my point is you should be looking for problems elsewhere in my experience. Of course measure them. I can’t tell if they are 2.7 ohm or 27k or 270k ohm. If 270k they could be plate load.
 
Last edited:
The outputs are Telefunken and the EF86 input tubes are Mullards - so if I was to try to get a set of each today I would need to sell a kidney.

I have swapped and changed various parts and it looks like the Telefunkens. There is a hum on one channel but its pretty low level - so hopefully theres a bit of life in the Telefunkens yet.

I tested all the remaining old resistors and at least half of them were slightly or largely out of spec - one by 50kohm. I will replace all of these so I know where I am with the passive components in there at least.
 
OK so I have tested all resistors and replaced those out of spec. The last ones are these - a bit of a conundrum. They appear to be on the b+ supply - note that they are yellow-violet-black with 10% tolerance. Thats makes them (supposedly) 47 ohms. The issue is that after testing both of them - they are both exactly 70 ohms - I have tried two different testers to get the same result. If they were meant to measure this, then the color code should be violet-black-black.

Would it be possible for two resistors to drift to the same value? I am testing with one leg lifted and out of circuit.

The other thing is Im not sure where these are in the circuit - They are both right next to the bridge rectifier in the circuit. You may need more info on this part - I understand.....

cheers

IMG_4069.jpg IMG_4070.jpg Fountain1010.gif
 
Not impossible. If they measure bad and the meter is working correctly, I'll vote for bad parts.

Schematic comes out too small to read values, can't tell you where they might be. You'd have to trace it out and see what they connect to. Also not impossible that the schematic isn't going to be an exact match for what your amp has. I've seen that more than once, either because someone modified it or there was a production change along the way.
 
If a resistor measured in-circuit is reading low you should disconnect one end of the resistor and re-measure it (out-of-circuit). This is to make sure there is no parallel path which could make it look like the resistor is out-of-spec low. Any parallel path will make it look like the net resistance is lower than its' actual value.

If a resistor measured in-circuit is reading high then there is no need to disconnect one end - it is out of spec high and its' value is "as measured".

If the resistor is measured in-circuit and reads within tolerance then you are good to go. There is no parallel path and its' value is "as measured".

I did this recently for a Fisher tube amp and made a spreadsheet. I listed expected and measured resistances and I also listed which resistors I had to disconnect to measure. Many can be read in-circuit, but is nice to share which ones can't be measured in-circuit to assist others in working on the same model. It makes a nice week-end project. It took me a few hours to do and it was worth it.
 
Heres an improve schematic. Im still not majorly clear on what I should do here......I cant see any 47 or 68/70 ohm resistors marked on here either....i might be best to leave these in place?

cheers

Fountain1010.gif
 
Last edited:
It sounds like you aren't able to read schematics and follow that with parts in an amplifier. That makes it a bit hard to adequately faultfind a vintage amp, as circuit voltages should also really be measured, as they may be askew without you knowing it.

If you can show some more photos of the parts nearby, and the terminals that the resistors connect to I think it would become obvious to forum viewers.

There are also safety concerns with any vintage equipment, ranging from gross to benign. Safety usually starts at the AC mains input, but adding a fuse to that section (if there really isn't one) is not recommended if you aren't competent to do that sort of work.
 
I would replace the parts with new ones of the same value. They are clearly off value if they read 70 ohms. Stripes say 47 ohms.

Pay little mind to what is on the schematic, go by what is in the amplifier if it appears to be original.
 
Couple thought for consideration.

47 ohm or 70 ohm. Really not enough difference there to really worry about.

Hum follows tube. Tube is bad or pins are dirty.
I’ve seen loose tube sockets and dirty tube sockets hum.

Hum in general may be rectifier or filter caps. Maybe look to the power supply.
 
I can see filaments on one side of the ECL86's glow bright red for a few seconds before calming down again.....
Filament flash is common in European tubes. It's harmless. American tube companies generally have the oxide coating on the heater wires right down to the welding points, Euros don't. So, because the heater has a low resistance at turn on (i.e.-when cold), the uncoated area will light up until its resistance rises and it stops. It's alarming to see, but not worrisome.
 
Back
Top Bottom