Help in determing bias on Blue Jr. Mark 1

[Mick Bailey]

Re-reading the posts and it also seems to me that if the amp is working properly with a decent volume something must be wrong with the voltage measurements across the OT primary. Are your probes sharp and are you scratching through any oxide layer with your probes? I have needle-sharp stainless Fluke probes and even then I can get very low measurements if I only touch an oxidized area. If in doubt, discharge the amp and clean up CP1, CP2 and CP3 and re-check.

If the voltages are correct, then this may point to really tired, low-emission EL84s. How old are these? These amps run really hot from the factory and as a result they're hard on output tubes.

Have you measured the amp's output just to be sure its pushing out full power? If it is, then the tube voltages and currents will all be OK.

[ArturoQ]

Re-reading the posts and it also seems to me that if the amp is working properly with a decent volume something must be wrong with the voltage measurements across the OT primary. Are your probes sharp and are you scratching through any oxide layer with your probes? I have needle-sharp stainless Fluke probes and even then I can get very low measurements if I only touch an oxidized area. If in doubt, discharge the amp and clean up CP1, CP2 and CP3 and re-check.

If the voltages are correct, then this may point to really tired, low-emission EL84s. How old are these? These amps run really hot from the factory and as a result they're hard on output tubes.

Have you measured the amp's output just to be sure its pushing out full power? If it is, then the tube voltages and currents will all be OK.

Thanks and I do think corrosion helped with the instability of the readings.

[ArturoQ]

If you don't want to cut the traces, your AM530 is quite capable of measuring current, you just have to be very careful, and remember to disconnect the probes at the meter end as soon as you are done (to avoid damage elsewhere if you forget it's set up for current).
You would need to disconnect the CP1 connection and insert the meter in series, then repeat for CP3. Meter set up for DC mA measurement.

Hello and finally I have some readings. Between the corrosion and some kind of meter problem the CP3 > CP1 stabilized @ .028 Adc as did the CP3 > CP2 measurement but only more fleetingly. For some reason the mA reading was ..... problematic.
Thanx for the insight.

[g1]

0.028A at 335V on the plate puts your idle dissipation at about 9W. For a 12W max. tube that puts you about 75% plate dissipation.

[Chuck H]

0.028A at 335V on the plate puts your idle dissipation at about 9W. For a 12W max. tube that puts you about 75% plate dissipation.

Which is conservative for EL84's at 335V by modern standards regardless of what the spec sheets say.

[ArturoQ]

0.028A at 335V on the plate puts your idle dissipation at about 9W. For a 12W max. tube that puts you about 75% plate dissipation.

Well that is interesting. I am going to have to try again, just to confirm. 9W seems quite conservative to me also. Other than re-solder the tube board socket pins, tubes and a new speaker it is stock.I am surprised. It is way loud.
Good evening

[ArturoQ]

Which is conservative for EL84's at 335V by modern standards regardless of what the spec sheets say.

Hello and thank you for your input. When you say modern standards, are we talking about the 90's or today current standards?

[Mick Bailey]

In many guitar amps EL84s are run over their original 50s spec sheet values which give conservative values based on long life and freedom form distortion. Over the years there's been a tendency to run tubes at higher plate voltages and dissipation to get a good sound. Consequently tubes have a relatively short but glorious life compared to their use in (say) a radiogram. Dr.Z, Mesa, Marshall and others really punish those tubes - much more so than even vintage AC30s. The real escalation possibly started with Traynor in the 70s.

Again, how old are your output tubes? Old tubes suffer from low emission, and therefore lower conduction through the tubes, giving skewed idle current values.

[Helmholtz]

Don't mix up fixed bias and cathode bias.
To me everything above 70% with fixed bias is hot.
Completely different with cathode bias.

[ArturoQ]

In many guitar amps EL84s are run over their original 50s spec sheet values which give conservative values based on long life and freedom form distortion. Over the years there's been a tendency to run tubes at higher plate voltages and dissipation to get a good sound. Consequently tubes have a relatively short but glorious life compared to their use in (say) a radiogram. Dr.Z, Mesa, Marshall and others really punish those tubes - much more so than even vintage AC30s. The real escalation possibly started with Traynor in the 70s.

Again, how old are your output tubes? Old tubes suffer from low emission, and therefore lower conduction through the tubes, giving skewed idle current values.

Good Day,
They are JJ and have to be maybe over 5 years. For my uses they and the amp sound fine. It is the inability to get a proper reading I am butting my head against. Dug out my old Simpson 260 and trying to remember how to do the amperage tests....

[ArturoQ]

Don't mix up fixed bias and cathode bias.
To me everything above 70% with fixed bias is hot.
Completely different with cathode bias.

Hello
From what I've garnered, 70% is the general consensus. That is from what I've seen online. Thanks for the information!

[Mick Bailey]

Don't mix up fixed bias and cathode bias.
To me everything above 70% with fixed bias is hot.
Completely different with cathode bias.

Many of the cathode biased amps run tubes too hot, too. For example, Dr.Z Maz 18 amps eat tubes. By contrast the tubes in a Marshall 2020 (fixed bias) idle at 14W and last longer in that amp than the Dr.Z.

All of the unmodified Blues Juniors I've had in for repair run close 12W idle dissipation, sometimes a higher - made worse in the UK by Fender's insistence on configuring PTs for 230v. Many have been run for years like that, as that's how the factory sent them out until they went over to cathode bias on the much later boards. It would be unusual to find an unmodified amp that's running a more sensible 75% with good tubes. Adding a bias trimmer or changing the fixed resistance values is a really common modification.

[Helmholtz]

Many of the cathode biased amps run tubes too hot, too. For example, Dr.Z Maz 18 amps eat tubes. By contrast the tubes in a Marshall 2020 (fixed bias) idle at 14W and last longer in that amp than the Dr.Z.

I have absolutely no respect for designs that vastly (say by more than 20%) exceed tube limits.
Just don't believe that redhot plates sound "better".

Cathode biased amps can be biased to 100% (or even 120% with reduced tube life) because here plate dissipation tends to drop with increasing output.

Fixed biased on the contrary typically causes a strong increase of plate dissipation (often around 30%) at medium output.
So to me a 70% upper limit seems to be a good rule of thumb with fixed bias.

...- made worse in the UK by Fender's insistence on configuring PTs for 230v.

Now that's seems to be a homemade UK problem.
AFAIK, legislation requires products to be specified/rated for the official nominal voltage of the country.
As nominal line voltage in the UK is 230V, you can't blame Fender.

See here (bottom of page):
https://www.spenergynetworks.co.uk/p...e_changes.aspx

[Chuck H]

As Mick noted, many manufacturers run EL84's hot and at higher than spec voltage. Regardless of bias method. Matchless may be the worst? There IS a difference in tube behavior between running them this way and more conservative operating conditions. EL84's run more conservatively have that famous creamy but bristly distortion with some intermodulation. At higher voltage (like 400Vp) and hotter bias (like 100%) they really do get harder and tighter sounding in their distortion character. More like little, big bottles. Tube life is measured in the hundreds of hours rather than thousands. That said...

I do also agree with Helmholtz that there's no need to abuse them such as many manufacturers do. I've worked a lot with this tube type and my favorite amp has 365Vp running at 90% using cathode bias. If it matters I'm using an 8k load. Plenty hot to be sure but not as bad as some. And a sturdy pair will give me a couple of thousand hours in a combo. They typically go microphonic before failing electrically. And they sound penty high and tight without having to put 400V on them or bias them over 100%.

[Mick Bailey]

I'd also volunteer that Matchless is the worse for over-running tubes in cathode bias. I've had some of their 'problem' EL34 amps running up to 120% and these suffer more in these amps due to the insulated clamps around the bases - I think they have a Nomex or similar lining. One I repaired had bases that had crumbled to nothing due to heat. In a studio environment (especially where the amps are idling all day and not put into standby) an unmodified amp will last just a few weeks on a new pair of output tubes.

2000 hours is also my own rule-of-thumb for an EL84 amp with decent tubes. Some of the Soviet EL84 equivalents hold up really well under more extreme conditions and I used to recommend and fit the Sovtek EL84M, which was their in-house labelling for tubes which originally came also with Cyrillic part numbers. Sadly, they became way too expensive as they became more popular.