Source for "economical" 10" drivers to rebuild HR-40?

[tpappano]

I find myself now owning a HR-40, the reverb unit is wrecked, almost all the drivers have serious cone issues and the amp can barely squeak 8) The amp I should be able to fix but I was wondering if there was a source for reasonably priced (cheap!) drivers. I don't want to spend real money fixing this thing up, I mainly got it because I was intrigued by such an early example of bi-amping and the 9-driver array brings back memories from the fifties of "Sweet 16" systems that were popular among early stereo fans.

Thanks!

[David Anderson]

You can find Celestion Ten 30 10" speakers for ~$35. They are made in China. You may be able to find cheaper Chinese speakers on eBay.

More generally, however, if the HR-40 is that condition, I'd have to question the wisdom of trying to restore it. They sound OK in larger spaces, but nothing special. The only way they were built that way was because the options for getting a lot of bass at the time were limited.

If you want to put effort into a Hammond Tone Cabinet restoration, I'd at least start with a PR-40.

[tpappano]

Thanks, David! Yeah, the developing consensus is that this HR40 is probably best suited for use as firewood 8) I found that Celestion speaker yesterday for $35 at Guitar Center, and there does not seem to be anything else cheaper available. My plan at this point is to simply patch the existing drivers as much as possible with glue and paper, repair the amp, and enjoy whatever amusements the thing can deliver until something more interesting falls into my lap. I seem to be getting a lot of satisfaction lately from repairing trashed amplifiers. It may be a developing obsession!

[bnelson218]

Check out partsexpress.com for a large speaker selection.

[tpappano]

Thank you! Parts Express has some for $10.76, cheap enough!
All the drivers in this thing were trash, some even had shorted voice coils. The amp was an easy fix- three carbon comp resistors simply weren't anymore 8). You guys were right on the nail about these old resistors being sketchy.

[David Anderson]

It's odd to find resistors opened up in these amps. I've found them stretched/increased in value, but never gone open. Hammond tended to source fairly high-quality parts.

All the paper capacitors in metal cases should be replaced, but that probably goes without saying.

It is odd that the 10" speakers were that badly damaged. What happened to them? Considering that it's nine speakers sharing 20 Watts, that's only 2.2 Watts per driver.

[tpappano]

The bad resistors were both of the 180k screen resistors for the 6SJ7s. One was completely open and one measured about 3 meg. The too low screen voltage was shutting down the 6SJ7s, preventing any output. The other was a cooked 5.6k 6SN7 grid resistor. Not sure how that might have happened but it was effectively shorting one side of the balanced input.

Turns out the speakers mainly just have a lot of cone damage. The ones I thought were shorted (the 12" ones) were not. Someone had been in the top of the cabinet fooling with the wiring and mucked it all up. Other than cone tears, they spoke. I did a quick check of the other resistors and caps. The resistors were within tolerance and of course the paper caps had drifted up but not as much as I would have thought after reworking my M3. I'll change them out, there aren't very many.

Since the replacement speakers seem to have fairly decent power ratings, for fun I'm tempted to drive them with one of my Crown CT200 amps with an active crossover 8)

[David Anderson]

Yes, screen resistors seem to take some wear and tear.

The 5.6k grid-leak resistor failure on a 6SN7 grid is strange since it's under virtually no load and mainly functions as a high-pass filter in conjunction with the 0.15uF capacitor. And CC resistors typically fail open, not shorted. Weird.

Be careful with the load with a SS amp. Nine 8Ω speakers in parallel results in a 0.89Ω load. Not many SS amps are rated for less than a 2Ω load. I'm not sure about the Crown CT200 specifically.

The main problem with the paper capacitors in these amps, specifically, is not necessarily the value drift, but the DC current leakage. The paper capacitors in hard brown plastic (?) cases are often, surprisingly, still good, but the earlier metal cased ones tend to be leaky.

[tpappano]

Yeah, a puzzler for sure on the resistor. Perhaps someone mis-wired a cable and 120 vac was applied to the audio input pin, but even that would have been less than 1/2 watt. Maybe a 6SN7 with an internal short, who knows.

I figure wiring the nine speakers series-parallel to present 8 ohms to the amp, which brings me to a question that has been bugging me. You look at the Hammond designs and there is almost nothing that can be criticized from an engineering standpoint. It seems a lot of effort went in to saving money by not having having unnecessary components, for example using a single grid resistor to serve two tubes. I must admit to being completely baffled as to why the 4th version of this amp was set up for driving the nine in parallel instead of series-parallel like a previous version. The lower impedance just means higher currents in the speaker wiring and more loss, although with such a puny amp it wouldn't matter that much, it still seems weird to me 8)

[picothinker]

I have a K-40, in MIssouri cheap. It was removed from service working.

[David Anderson]

I must admit to being completely baffled as to why the 4th version of this amp was set up for driving the nine in parallel instead of series-parallel like a previous version. The lower impedance just means higher currents in the speaker wiring and more loss, although with such a puny amp it wouldn't matter that much, it still seems weird to me 8)

There was a discussion of this on a tech page I'm a member of in the last year, specifically why series-parallel wiring always seems to sound different from full-parallel wiring. It was a long discussion. I can only go by what I've heard, but someone told me that the earlier series-parallel wired HR-40s sounded even worse than the full-parallel version.

Perhaps it simplified manufacturing, but you can wind an output transformer for a wide range of impedance matching.

The best thing I can say about HR-40s is that they sound OK (just OK) in a large space like a church sanctuary, which is what they were designed for. Highs are muted compared to the bass. They don't sound right to me in a smaller room. The HR-40 and JR-20 are my least favorites of HTCs I've heard.

[tpappano]

Hmmm... Thinking about this some more, it may be "damping factor". With all drivers in parallel, any individual speaker sees as its source impedance the parallel combination of all the other speakers and the amplifier, which would give a relatively high damping factor. With a 3 x 3 series parallel arrangement, any driver has two other drivers in series with it, increasing the source impedance and reducing the damping factor. So, in theory the all in parallel arrangement would provide the tightest control of cone movement and better sound. Then on the other hand, should it matter when all the cones are moving in the same direction at the same time 8) I guess only one way to find out...

[David Anderson]

So, in theory the all in parallel arrangement would provide the tightest control of cone movement and better sound.

I've run across this conclusion in many audio contexts, and full parallel is the arrangement you see most often, the exception being some Marshall 4 x 12" cabinets.

The only other place I've seen series speakers used in guitar amps is in some budget off-brand amps and a couple of Gibson models that were adapted from single-speaker models.