Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BruceWA on September 25, 2022, 04:46:14 pm
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Been bringing a 1971ish Montgomery Wards (Vaco) GIM 9171A back to life and could use help on a couple of things.
1. Reverb Tank. Anyone know the correct tank? I have wired in leads and am using an insulated in and grounded out (4EB2C1B). Works but I get hum from the circuit.
2. This amp has 2 OTs and uses stereo out (TRS) grounded jacks. Having trouble locating them! Originals are failing.
3. Lights. Original lights and circuits are missing. Any thoughts on updated version? Thinking about Electro Luminescent tape but don’t want to add a rectifier and drop voltage.
A couple of pics later when I can shrink them down so you can see what’s on the bench.
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last week someone dropped off a gim-9131 and gim-9151 with clipped two prong power cords for me to have so i'm interested to hear about the reverb tank. for the output, the bass amp uses two transformers as well. i was planning to just wire them like a dual ot jtm45/100
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Uphill battle for both of us thetragichero. My schematics of the JTM45 shows one OT. Never worked a dual OT before. Will search for other schematics.
As for the tank, the input is not grounded, that limits usable tanks. My issue is impedance matching. Only so many tanks in stock as I am a one-man shop. Will update my post as things continue!
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Searched for a schama - I see a 10Ω resistor in parallel with the tank, therefore the load seen at the reverb tank driver transformer's secondary is <10Ω, so would assume that the tank input Z would be 8 to 10Ω. The hum source in the reverb loop could be the grounding scheme of the tank input/output jacks.
--Pete
Schema I referenced:
https://el34world.com/charts/Schematics/files/Wards_airline/Airline_wards_gim_9171a.pdf (https://el34world.com/charts/Schematics/files/Wards_airline/Airline_wards_gim_9171a.pdf)
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Thanks Dummload for validating my thoughts. Ordering a couple of 4Axxx tanks with various output (return) values in this weeks parts order for the shop. Shipping is a killer!
The rest of the amp is sounding as I expect it to; very mid-60s Fendrish.
Still deciding on how to address the output. Put OTs in series or keeping it stock and building a 2-12 cab to match. Leaning towards the later.