Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BrianS on January 22, 2021, 03:44:46 pm

Title: Wards GIM 9171A...circuit analysis?
Post by: BrianS on January 22, 2021, 03:44:46 pm: https://el34world.com/charts/Schematics/files/Wards_airline/Wards_airline_gim_9171a.pdf (https://el34world.com/charts/Schematics/files/Wards_airline/Wards_airline_gim_9171a.pdf)
Just had one of these on the bench...no serious problems and the amp sounds really good. A couple things I found odd:
The four input jacks are not shorting jacks and just sit there, open when nothing is plugged in, making the amp really noisy. I replaced all those with shorting jacks and a common ground buss and the amp quieted down to an expected level.
The other really big oddity is that the amp uses two output sections, with two separate output transformers. The outputs of these transformers are summed together at the output jack. Why on earth would a company do that? It just seems really odd.
Just wondering if anyone else has ever seen one of these or might have any insight on this design.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: thetragichero on January 22, 2021, 03:56:31 pm: so like the dual ot design of the jtm 45/100?
I'm not super well-versed on marshall low but i imagine if one has a surplus of the smaller transformers and wants MOAR POWER that's certainly a way to do it
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: sluckey on January 22, 2021, 04:06:59 pm: Quote from: BrianS on January 22, 2021, 03:44:46 pm
The other really big oddity is that the amp uses two output sections, with two separate output transformers. The outputs of these transformers are summed together at the output jack. Why on earth would a company do that? It just seems really odd.
Just wondering if anyone else has ever seen one of these or might have any insight on this design.
The outputs are never summed. TRS jacks are used to maintain separation all the way to the speaker cab. Why? Probably to compete with Sears. :laugh:
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: DummyLoad on January 22, 2021, 04:13:51 pm: ex. you sell 1000 units of the 35W model for every 200 units of the 70W model. drives the price down if you by 1200 pieces of the 35W part rather than 1000 35W part and 200 of the 70W part.

yes, seen/repaired/owned a few - some ex. are: silvertone 1485 twin; grestch 6169 fury; grestch chet atkins models.

--pete
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: BrianS on January 23, 2021, 09:27:38 am: Quote
The outputs are never summed. TRS jacks are used to maintain separation all the way to the speaker cab. Why? Probably to compete with Sears.
Quote
Ah, I see that now. So when testing this amp with a standard speaker plug, I was essentially grounding out the "top" output section...
Could I "sum" the two outputs together by using a standard TS output jack? Looking at some other info on this site, it looks like I could tie the transformer secondaries in series and then be able to use an 8ohm speaker. I better check with the customer to see if he has the original cab first...I don't think he does.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: BrianS on January 26, 2021, 08:27:36 am: Tied the output secondaries together in series. Works good, amp is usable with any 8ohm speaker now. Amp sounds great and overall has really good build quality.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: HotBluePlates on January 27, 2021, 08:08:07 am: Quote from: BrianS on January 22, 2021, 03:44:46 pm
... The other really big oddity is that the amp uses two output sections, with two separate output transformers. The outputs of these transformers are summed together at the output jack. Why on earth would a company do that? It just seems really odd. ...
Quote from: sluckey on January 22, 2021, 04:06:59 pm
The outputs are never summed. TRS jacks are used to maintain separation all the way to the speaker cab. Why? Probably to compete with Sears. :laugh:

Spit-balling:

- Couple smaller high-production transformers cheaper than a single high-power output transformer.

- No space in the head cabinet to fit a single OT (https://i.imgur.com/4dRS2HL.jpg); 2x OTs allow the iron to be spread out.

- ~~Available Wards speaker cabinets would be blown by 4x 6L6;~~ 2 output sections + TRS forces ~~applying only a safe amount of~~ half power to each ~~cabinet used~~ set of speakers.

Of course, there's no warning on the chassis that a Y-cable is needed. I wonder if there ever was a flyer or manual for this amp saying a special cable was needed to break out to the separate cabinets.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: sluckey on January 27, 2021, 09:45:53 am: Quote from: HotBluePlates on January 27, 2021, 08:08:07 am
Of course, there's no warning on the chassis that a Y-cable is needed. I wonder if there ever was a flyer or manual for this amp saying a special cable was needed to break out to the separate cabinets.
Y-cable??? The schematic shows a single speaker enclosure with TRS jack and all drivers.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: HotBluePlates on January 27, 2021, 01:24:11 pm: Quote from: sluckey on January 27, 2021, 09:45:53 am
Quote from: HotBluePlates on January 27, 2021, 08:08:07 am
Of course, there's no warning on the chassis that a Y-cable is needed. ...special cable was needed to break out to the separate cabinets.
Y-cable??? The schematic shows a single speaker enclosure with TRS jack and all drivers.

Only wild speculation on my part.

Though the schematic in the OP doesn't show any speakers or cabinets, just the TRS jack.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: sluckey on January 27, 2021, 01:29:28 pm: Look again. The speakers are hiding in the middle of the schematic, directly below V3-B. Not where you or I would have drawn them. :wink:
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: Williamblake on January 27, 2021, 01:44:22 pm: So T1 is tip and T2 is ring and sleeve is OV on each speaker jack. I dont see sense in that. ZAnd i dont understand the switching on the lower jack.

// So the switching in the lower jack is protection but you still need a fancy cable for your external speaker.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: HotBluePlates on January 27, 2021, 01:46:30 pm: Quote from: sluckey on January 27, 2021, 01:29:28 pm
Look again. The speakers are hiding in the middle of the schematic ...

And here I was thinking my glasses prescription was up to date!! :l2:

So the TRS is all about only feeding 1/2 the total available-power to the leg with the horn, huh?

Oh well... everyone likes a Zither.

(https://i.imgur.com/6lXyKADm.png)
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: sluckey on January 27, 2021, 01:58:44 pm: Who can say about Monkey Wards. Probably just an advertising ploy to compete with Rears and Sorebutts 1485. It would be interesting to read/compare the page from the catalogs of both those amps. I bet both amps came from the same factory.
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: BrianS on January 27, 2021, 07:11:35 pm: Well, now I don't feel so bad about missing the speaker cab when I first looked at the schematic. Sometimes it's just too easy to make assumptions, which is dangerous business for a repair tech!Here's a gut shot of the amp before I did any work on it. My "after" pics are still on my phone...All in all I think it's a pretty well laid out amp & was easy to work on. All the tubes were RCAs and the reverb & trem sounded really good.You can see the standard "open contact" (or whatever you'd call them) input jacks. Changing these out to shorting jacks made a HUGE difference in the noise level of the amp.

(https://scontent-msp1-1.xx.fbcdn.net/v/t1.0-9/140421406_3684605744940480_2716839243091494004_o.jpg?_nc_cat=103&ccb=2&_nc_sid=a26aad&_nc_ohc=myCge-zSwboAX9Yhv1Z&_nc_ht=scontent-msp1-1.xx&oh=48b202d1b2769136abadbbd7923dc7f7&oe=6036AF95)
Title: Re: Wards GIM 9171A...circuit analysis?
Post by: sluckey on January 27, 2021, 07:19:35 pm: Much neater than any Silvertone I ever saw.