Any danger in bi-amping Fisher tube amps?

[thornev]

Since my 500-C has 2 speaker selections, I thought I could bi-amp or bi-wire my AR-2AX speakers which have the ability to separate the woofer driver from the mid-range and tweeter drivers. I was curious if there would be any improvement to the sound. I connected the speaker1 outputs to the woofer driver lugs and the speaker2 outputs to the mid-range and tweeter driver lugs. Since the AR-2AX only has 3 lugs on the speaker, one of the common leads from the Fisher is not connected to anything.

It works, but I'm concerned about the risk. I've been told that the 500-C doesn't have a common ground and because of that it is possible for something bad to happen either to the amp or to the speakers. I'm unknowledgeable about it though. Can anyone shed any light on if it's OK ? The sound seems pretty good, but at one point while I was listening, I thought I heard the woofer driver cut out so I turned the receiver off immediately. Later testing proved everything is OK. I don't believe it's genuine bi-amp'ing because the 500-C is not really 2 separate amps.

On another topic, I'm thinking about adding a fuse to the speaker out leads from the Fisher just in case a power spike were to occur. I've figured out the correct amperage. I believe I would install the fuse on each positive lead from the Fisher to each speaker. Does that sound like the correct approach?

Thorne

 

[larryderouin]

You're not bi-Amping from the 500C. To Bi-Amp you'd need 2 separate amps with setups to amplify only certain frequencies (ie: one amp for Bass and Mid bass, one amp for mids and highs). Right now you've got a FULL Range amp driving 2 sets of speakers (with the loads split in the 2AX's). Granted the X-OVER Networks in the 2AX's will separate out the low's (for the mids and the highs) and maybe the mids and the highs for the Thumper, but it's not ideal. Plus running them split presents 2 - 8 ohm loads to the transformer effectively dropping the load to approx 4 ohms (or a roughly 2 ohm load if the 2AX's are 4 ohm nominal). So you need to compensate for that too. Honestly, I'd put it back as original and run it that way. You could get a couple of old Motorola 3 channel 6BQ5 amps and make them 2 channel ( low and one high) then use the 2ax's in split mode.

 

[gadget73]

thats bi-wiring. If the speaker selector will output on both at the same time, it won't be any different vs a single lead like you are using now. No harm to the amp.

 

[opnly bafld]

Granted the X-OVER Networks in the 2AX's will separate out the low's (for the mids and the highs) and maybe the mids and the highs for the Thumper, but it's not ideal. Plus running them split presents 2 - 8 ohm loads to the transformer effectively dropping the load to approx 4 ohms (or a roughly 2 ohm load if the 2AX's are 4 ohm nominal). So you need to compensate for that too.

What is the difference between running one pair of wires or two from the amp, when the internal wires that go to the x-o are already split?
The split just happens sooner (at the amp) with bi-wiring.
If a speaker is rated at 8 ohms (nominal) it doesn't matter if the the input to the x-o is split or not, still an 8 ohm load.

 

[gadget73]

Now you're into the land of contentious arguments

Electrically speaking, its still 8 ohms. The amp is not going to see anything different.

Where the argument comes in is usually about what sort of wire is best for what frequency range. In bi-wire rigs, you'll occasionally see some big fat wire for the woofer, and some other sort of wire for the highs, with the idea of higher strand count or something being better for highs due to skin effect. Once in a while you'll also hear arguments about the back EMF from the woofer not affecting the tweeter as much if you split the feeds as far from the woofer as possible.

Or something. Personally I think its much ado about very little. Skin effect exists, but frankly its not enough to fuss over at 20 khz.

 

[opnly bafld]

FWIW
I asked the rhetorical question in reply to the bold quote above it (changing an 8 ohm speaker to 4 ohms), it was not meant to be a philosophical or theoretical question.

 

[thornev]

A twist in the story and a new question... I decided to put the speakers back to normal - one pair of leads per speaker. For some reason the Fisher started sounding weak and that's a crime. I now have both my Marantz and Fisher hooked up to a "speaker distribution center" (https://www.ebay.com/i/222141897390?chn=ps) so that I can choose to listen to either receiver through either my AR or B&W speakers. However, the Fisher still sounds weak to the point where I have to turn it up almost twice as loud to get the same volume as without the speaker distribution device.

Now I know from conversations on classicspeakers.net that there is some concern about hooking up the Fisher, which is allegedly a dual-ground system, to a device that also has the Marantz hooked up to it. And I suspect that that has something to do with why the Fisher is sounding weak. My question is to understand why, when having these 2 receivers connected via the speaker center, does the Fisher sound weak (the Marantz doesn't - it sounds fine). And is there anything I can do to prevent this Fisher's sound from weakening, other than keeping the Fisher detached from the speaker center?

Interesting note... I measured the 7591A Fisher power tubes through the IBAM and found that after connecting the Fisher to the speaker center, the amperage dropped from 34mA to 31mA. I had to adjust the bias to get the tubes running at 34mA again. But the sound is still weak and even more distorted than before.

Thorne

 

[tcdriver]

A twist in the story and a new question... I decided to put the speakers back to normal - one pair of leads per speaker. For some reason the Fisher started sounding weak and that's a crime. I now have both my Marantz and Fisher hooked up to a "speaker distribution center" (https://www.ebay.com/i/222141897390?chn=ps) so that I can choose to listen to either receiver through either my AR or B&W speakers. ...

Try hooking up the Fisher directly to the speakers. If it sounds normal (not weak), the switch box may be the problem.

 

[thornev]

tc - I know having the Fisher standalone is fine. What I don't know is if the speaker box is the problem and, if it is, why? What's the electronic explanation for why the Fisher volume is lower and more distorted. That's what I'm asking.

 

[1rebmem]

Thorne,
Just so I can follow along....the Marantz is a solid state unit?

And when you swap the Fisher and Marantz going into the TEC-905B, the Fisher always sounds bad and the Marantz always sounds fine?

 

[thornev]

Jef... Yes, Marantz is SS. There's no swapping the receivers per se. The receivers and speakers are always hooked up to the speaker box. I just press buttons on the speaker box to route the receiver to which pair of speakers I want to hear. The receivers are never both on at the same time and I never have both pairs of speakers playing at the same time. Someone suggested that through the ground wires to the speakers that both receivers are always sharing the ground which might have something to do with why the Fisher is affected by this setup. Thorne

 

[1rebmem]

There's no swapping the receivers per se. The receivers and speakers are always hooked up to the speaker box

Right, but what I meant is, if you swap the receivers at TEC-905B inputs, the Marantz works on both input locations and the Fisher doesn't work on either?

 

[thornev]

Jef... I tried hooking the Fisher direct to speakers and through the speaker box. There was no difference in volume nor overall sound quality. This result doesn't make sense to me because it contradicts my previous claim that the Fisher's sound is degraded when hooked up to the speaker box to which the Marantz is also connected. So now I have to consider other variables such as the FM radio signal since that was the source when I originally made these observations. What may have influenced my observation was comparing the Fisher sound to the Marantz sound - the Marantz sounds cleaner than the Fisher. Comparing solid state to tubes is probably not a good test. Two different sounds. I added a new variable (which was not present when I made my original observations) - a voltage regulator. The voltage to my house varies between 115 and 124. The regulator attempts to keep the voltage at 110.

Bottom line now is that everything appears to be OK. Sigh. Thorne

 

[tcdriver]

tc - I know having the Fisher standalone is fine. What I don't know is if the speaker box is the problem and, if it is, why? What's the electronic explanation for why the Fisher volume is lower and more distorted. That's what I'm asking.

Yes, it would seem that the switch box is the problem. The Fisher output transformer COM pin is not the Fisher GROUND. When connected to the switch box the Fisher COM pin is grounded shorting out part of the output transformer secondary. What you could try is connecting the Fisher 4 ohm tap to the switch box negative or common connection and the 8 or 16 ohm tap to the switch box positive. That would eliminate the short circuit condition.

 

[thornev]

Thanks, tcdriver. I'm apprehensive about using the 4 and 16 ohm speaker outputs as the speakers are 8 ohms. The difference in impedances I would think surely will produce other undesirable results. Is it unusual for the chassis ground and the speaker COM to be different? Since the Marantz is not affected, I assume ground and COM are the same on it ?

I guess since the speaker box is the source of the problem, to solve my desire to be able to choose which speakers to listen to, instead of a speaker selector, I should install an audio-source selector aka mixer which hopefully can connect/output to both receivers. So if I want to listen to my AR speakers which will be hooked up to my Fisher, I connect my CD player to the mixer and select CD player as input and Fisher as output and voila, I'm listening to my AR speakers and the Fisher at full power.

PS - I can do this with a mixer that has a headphone jack. The headphone jack connects to the AUX input of one receiver while the MAIN output from the mixer goes to the other receiver.

 

[larryderouin]

Thorne. The speakers on the 500c are grounded from the 4 ohm tap/end of the secondary. Chassis ground is NOT THE SAME on the FISHER. The Marantz grounds the spkrs to the chassis. Differences in grounding will cause all kinds of problems as I remember (and THAT is getting worse daily!!! OLD AGE IS A GOLD PLATED BIOTCH!) Not sure how to solve the problem with the FISHER vs. the Marantz. But I bet Dave would know.

Use the COM as the positive lead and the 4ohm as the Negative. Using the 4 or 16 ohm tap on an 8 ohm driver won't hurt it. You'll get a very slight improvement in SPL and possibly SQ in 4ohm. The reverse is true for SPL in 16 ohm, but SQ may improve slightly.

 

[thornev]

OK, I'll give it a spin, Larry, thanks.

 

[tcdriver]

One additional solution would be to use a 4PDT switch to connect the Fisher and Marantz receivers to the speaker box. The common terminals of the switch are connected to the speaker box. Each receiver has four speaker connections to + Left, - Left, + Right, - Right. In this configuration the - terminals of each of the amplifiers cannot be shorted together.

 

[Sam08861]

You could also connect both receivers into something like a Niles SPK-1. (I use this to switch between the Fisher and my SS receiver hooked up to the same speakers). You should be able to find these pretty cheap. (I usually see these going for $20-40 used).

How they work is they have internal relays, so no danger of having both amps short. A 5V wall wart plugged into one of the receivers switched outlets activates the relays, switching over the source.

The nice thing about this setup is not having to switch out wires.

 

[thornev]

Larry - Your suggestion worked. I used the 4 ohm lug. I don't know if there was any improvement as I don't know how I would measure it. Thanks again for your feedback. Thorne