Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jukelemon on March 07, 2016, 10:02:29 am

Title: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: jukelemon on March 07, 2016, 10:02:29 am

Hi all.

Sonically speaking...what would be the effect of using a 022921 Reverb Driver (8 ohm secondary) with a Gibson type tank which has ~1750 ohm input impedance?

I have a Gibson GA-19RVT Crestline using a 6c4 Reverb driver. The amp sounds fantastic sans the reverb - it is like the signal is hitting the tank way too hard. Not sure how else to describe it. It is like 50% nasty plate reverb with 50% spring.

When I examined more closely, I fouund a replacement reverb tranny BUT the stock tank.  Measurements on the tank confirm it is stck and looking for a ~1750 ohm input and therfore the basis of my question.

Thanks all

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: mresistor on March 07, 2016, 11:21:25 am

I think I would replace the reverb transformer with a Mercury Magnetics GA-RV to match impedance of the pan. Or replace the pan with a Type A pan.


http://www.mercurymagnetics.com/pages/mainframe.htm (http://www.mercurymagnetics.com/pages/mainframe.htm)


http://electricalfun.com/WorkbenchFun/reverb_tank.pdf (http://electricalfun.com/WorkbenchFun/reverb_tank.pdf)

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: mresistor on March 07, 2016, 11:29:12 am

https://www.tubesandmore.com/products/P-RMOD-4AB3C1B (https://www.tubesandmore.com/products/P-RMOD-4AB3C1B)


the cheaper alternative   I know that doesn't address your question of sonic qualities of a gross impedance mismatch, but it would solve your problem.

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: jukelemon on March 07, 2016, 01:28:49 pm

Hi.

Thanks.

Yeah, I would rather just get the tank.

Hoping someone can chime in on the sonic effects...

Thanks

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: PRR on March 07, 2016, 03:15:03 pm

There is ALWAYS an "impedance mismatch". Tank input Z varies over 10:1 across the audio range.

> Driver (8 ohm...) with a 1750 ohm

That's a way-huge-lot of mis-match. Power delivered to the tank will be way-way down. About 15:1 down on voltage, thus "volume". Like using 120V lamps in your 12V car. Dim to the point of uselessness.

Won't hurt nothing. Try it. But I think you will want another RT or tank.

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: jukelemon on March 07, 2016, 04:49:59 pm

Thanks PRR.

I didnt suspect that answer i.e. power will be down.

There is no loss of power per se.  The Volume is normal/not different with it on or off.

And the sonic quality is the reverse to me i.e. like the tank is being driven too hard.  The tank specs out fine...at least what other Gibson tanks spec at so I dont think the tank is the issue in this mismatch.

Hmm...now I am puzzled.

I ordered a matching A tank for the Fender type Reverb tranny so I can only hope it gets better. If it doesnt...I guess I am going to have to play with the circuit.

Thanks

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: PRR on March 07, 2016, 05:02:02 pm

The total amplifier power won't change.

The power of the reverb part of the signal will be down.

> like the tank is being driven too hard

I will guess that you have to smack the DRIVER too hard to get enough reverb to make a difference. It wouldn't be obvious which is driven "too hard" since driver and tank input coil are so intimate, and whatever distortion happens is further confounded by spring-ing-ing effects.

Yes, the way the world is going, especially since you have a Fenderesque low-Z transformer, a Fender type 4-10 Ohm tank would be the best fix today. While 1K-2K tanks *may* be around forever, we KNOW the Fender low-Z tanks will be available until we all have brain-implants and stop fooling with amps and tanks and such.

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: jukelemon on March 11, 2016, 10:08:46 am

I wanted to follow up in this in the event someone else has/will have the same issue or concern.

New tank arrived. Essentially a typical 2 spring, long decay Fender BF type.

The difference was hugely significant.  That sonic issue that I mentioned is completely gone.  The reverb is smooth and MUCH stronger than before. 

Whereas I used to have to get it to ~7 on the dial to hear it well in the signal I am at no more than 3-4 and much like a Fender amp.

When I say significant, I mean like night and day difference.

I typically run a 62 Reverb tank with my amps so my expectation point to spring reverb is pretty high.  Now, this Gibson is pretty darn close to sounding as good/close enough to play shows.

Thanks all for the comments

Title: Re: Sonic differences when there is an impedance mismatch between Reverb Tranny and
Post by: PRR on March 12, 2016, 08:07:50 pm

me>>  Dim to the point of uselessness.
you>  night and day difference.

Why are we using light analogies for reverb?

Glad you got it sorted.