How do you tell a customer...?

[dorrisant]

How do you tell a customer that the tone he was gettin' from his amp is the same thing that may cause other problems?

Example: I have a Gibson Thor on the bench that the customer swears has the bass tone that he has always looked for... It's his baby. It has this "perfect breakup" that "just works". He has been gigging quite a bit with it. He brought it in because it wouldn't power up. Maybe it was the outlet that wasn't working at the time or something... I don't know. It fired right up on the bench, checked the voltages, tested the tubes. Power tubes were a bit weak but should be just fine. Nothing here... I checked the cab. Two 10" 10Ω series connected speakers are shown on the schematic. What he has is two 10" 8Ω parallel connected speakers.

Measured the OT... 14:1. So a 4Ω load reflects 0.8k on the primary. The schem shows a 20Ω load which would reflect 4k on the primary. The OT doesnt look that beefy.

It sucks! He ain't gonna like this amp any more...

[Malcolm Irving]

May not be as bad as it sounds. There will be no problem with screen current at all. Idling plate current and dissipation will be fine (assuming the bias is OK), they are not affected by the low reflected AC load.
The only problem I can see (others might chip in here) is that for half the signal wave the plate current and dissipation will go above the limit. But that is compensated to some extent by going well below the limit on the other half of the signal wave.
If it hasn't blow the output tubes or the OT up to now, why not explain the situation to the customer and see if he/she wants to continue to take the risk?

[Stevem]

A 4 ohm load on that OT with the added secondary current that such makes WILL take out the OT at some point in time!

[cbass]

I agree it ain't livin long like that. Might get buy on an 8 ohm load though

[10thTx]

From Kevin O'Connor and London Power Amps FAQ:

Q: I thought impedance matching was critical. Some designers say the output transformer must be changed if you want to use different output tubes. That seems awfully expensive.

A: It is awfully expensive, and awful that such things would be suggested. There are two issues here, though; one is the notion of "impedance matching", and the other is simple design preference.

As stated throughout the TUT-series, speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.

With respect, 10thtx

[martin manning]

If this amp has been used for some time in its present configuration and it hasn't died yet, the situation obviously isn't all that dire. Speaker impedance is very nonlinear, and it tends to increase dramatically at bass frequencies, reaching 2-4x nominal. That might be the explanation for it's survival, and the reason that it sounds good. I'd explain that it doesn't appear to be wired according to the schematic, and that there might be some risk there.

[Littlewyan]

Or maybe don't say anything at first, get him in to play the amp set up properly and don't tell him until after hes played it. If you tell him before then he may automatically assume its going to sound bad and will stick with that opinion no matter how it sounds.

[martin manning]

What does the speaker wiring look like? Would it be easy for someone to connect it either way, or does it look like it has been deliberately wired for parallel?

[didit]

Looking at this schematic -- http://www.classictubeamps.com/schemati ... ematic.pdf -- and cannot see speaker nominal ohms indicated; series pair of 8s or perhaps 4s? Comments in thread below suggests 8s. However, O'Connor is correct. Within reasonable limits load matching is purely efficiency & distortion. Photos in this thread suggest OPT heft -- http://www.tdpri.com/forum/amp-central- ... e-amp.html. I'd be more concerned about bias adjustment.

Rare to see MI amp with full choke PI. Other interesting things also. Thanks for an introduction to something unique.

Best .. Ian

[dorrisant]

Martin - The speakers look like pro audio or PA variety. 451041 speaker doesn't help much on a quick search. Definitely not stock. Large gold spring terminals are very easy to reconfigure.

Ian - Yes that is the right schematic (also in the back of The Tube Amp Book, pp 109). If you enlarge it you can see two 10Ω speakers in series. What are you thinking about the bias?

[jjman]

The scheme in the link shows 10" (not 10 ohms.) Both in the scheme section and the parts list section. My The Tube Amp Book is 4.1 and I don't see it there.

[sluckey]

That schematic doesn't say 10Ω speakers. It says 10" speakers.

Here's a slightly better copy...

http://el34world.com/charts/Schematics/ ... SS-AMP.pdf

[xtian]

That schematic doesn't say 10Ω speakers. It says 10" speakers

Well, of COURSE you should specify the speaker DIAMETER on the SCHEMATIC! Duh.

[didit]

I read that as 10" too.

Experience with '60s era fixed but un-adjustable bias is just that it's often tad too hot. Sometimes value drift in the resistors, sometimes wall voltage shifts the divider up. Schematic calls for -34V. Could start by confirming that, if you haven't. Voltage really isn't the best gauge though so go on to confirm power tubes are happy regardless.

Best .. Ian

[dorrisant]

The scheme in the link shows 10" (not 10 ohms.) Both in the scheme section and the parts list section. My The Tube Amp Book is 4.1 and I don't see it there.

You are right. I'm officially going blind now.

I found this page: http://johnkvintageguitars.homestead.com/Thor.html

Its show 2-10" Jensen C10N 8 ohm speakers. Maybe the correct impedance won't be that much different after all.