Old Music Guy
Member
as much as I appreciate it, I think I'm good. It works for me in my setting. 12VDC is about as high I'll go theses days. Besides, I don't want my wife having to sweep up my ashes on the basement floor. She would be soooo pissed.
Question: Is a 30-watt amp head going into a 2x12 cab good enough for medium-sized gigs?
- Thread starter therevivalsband
- Start date
The stock Marshall has a universal transformer with taps for 230, 120V and 100V.
Schematic?
Dave.
A real tech would just do it and not tell anyone and there is nothing special with modding a simple voltage divider circuit. Even though I think a bias balance circuit as well as the bias voltage adjust circuit should be the standard for fixed bias push-pull tube output circuits. But of course amp manufacturers that sell tubes might get butt hurt over people using any 6L6 in the circuit. Even ones that don't match.
Old Music Guy
Member
AI generated schematic?
Old Music Guy
Member
And just how would you define a "real tech"?
Asking for a friend.
Ah! I think you understand OMG (just got that BTW!) I wonder how long it will take Himself?
Dave.
no, I used Falstad circuit simulator to quickly draw the schematic, then I just screen captured a section of that window.
I would have to define in in this context to keep this concise, but a well trained technician in a repair shop environment would always check bias after installing tubes. Even if they were the ones that suppose to be the correct ones accordingly to the manufacturer to use. Because even though the tube is the correct one, there is no guarantee it would bias the same. Storage and handling on the tubes plays a big factor on the differences acquired when the tubes were first manufactured, and tested to be in range to be sold as a discrete set for a particular unit. Some would just opt to use a different set of tubes in their inventory if they encountered a difference. Others would just calculate what size resistor(s) for the voltage divider to set the bias correctly and change out the resistors to match the new tubes. Either way, the end user gets their amp back from the shop working just as good as when they bought it.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
What stock universal Marshall transformer? never heard of it. will it fit in Marshall 1 rack space item?
The stock transformer on the 20/20 is not multi tapped. The channels A B are also ganged on one volume knob..
I have swapped the wires around on Korgs for JAP100V to US117, so I am familiar with what procedure you're talking about.
I pulled up the wrong schematic to look at the first time. Sorry about that.
The transformer in this unit, you change out. According to Marshall, the part number is 64591A. But since it doesn't show up on searches its a custom wound transformer that you have to contact Marshall to get it.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
Thank you. Yes, I see that too. 64591A. However I don't think it exists. Deltronics couldn't order it. Marshall tech support has no idea what part Im talking about. So they linked me custom transformer winding place in I think Poland..In the end I got nowhere.
4 years ago...........
4 years ago...........
Old Music Guy
Member
OK. Just wanted to know what you considered a "real tech".
I've worked in electronics for over 30 years. Some of the largest companies you've ever heard of: Defense, aircraft, space, photography, CGI, analog and digital test equipment design, R&D.
Glad to know I don't qualify as a "real tech".
Friend is satisfied.
Old Music Guy
Member
According to the schematic provided by drtechno, the transformer is a torroid. Hence, unlike "traditional" transformers, there is no "secondary" winding from which to tap off of.
Marshall did this because torroids are way more efficient than a traditional transformers and run very much cooler thus not wasting power in the form of heat.
There is plenty of information on torroids if you want to check it out.
Marshall did this because torroids are way more efficient than a traditional transformers and run very much cooler thus not wasting power in the form of heat.
There is plenty of information on torroids if you want to check it out.
Old Music Guy
Member
To confuse you even more, both traditional transformers and torroids are nothing but inductors.
In a traditional transformer, a voltage is applied to the "primary" winding. The primary winding is isolated from the "secondary" winding by an iron core. The voltage applied to the primary winding creates a magnetic field which "induces" a voltage on the secondary winding. The voltage on the secondary winding is a function of the ratio of the primary winding to the secondary winding, (This can further be explained with regard to "step up/step down transformers).
Torriods differ insofar as that the round iron core, in which the winding is wound, does not induce a voltage to a secondary. Thus, the voltage out is proportionate to the number of windings, etc, That is why there are no "taps". This is what makes torroids more efficient, there is no wasted energy (conservation).
In a traditional transformer, a voltage is applied to the "primary" winding. The primary winding is isolated from the "secondary" winding by an iron core. The voltage applied to the primary winding creates a magnetic field which "induces" a voltage on the secondary winding. The voltage on the secondary winding is a function of the ratio of the primary winding to the secondary winding, (This can further be explained with regard to "step up/step down transformers).
Torriods differ insofar as that the round iron core, in which the winding is wound, does not induce a voltage to a secondary. Thus, the voltage out is proportionate to the number of windings, etc, That is why there are no "taps". This is what makes torroids more efficient, there is no wasted energy (conservation).
Don't bash yourself in the head over it. Unless you have dealt with the particularities of these devices for years you wouldn't know some of the back shop practices. I've been a tech for many years in a lot of different sectors and there is always learning curves and practices. When I served in the USAF I was an ECS/electrics specialist and had to learn how to fix everything that had a wire or bleed air connected to it across the whole air frame inventory. Before that I repaired Consumer stereo equipment, restore antique radios and TVs and current consumer electronics until society changed and made everything disposable. I also repaired the tube amps from the local music store that subcontracted the work to me. But if you want me to component level repair a variable speed drive for a 3 phase motor I would have stumbling blocks and learning curves because I don't fix those things on a regular basis and don't know the common practices behind it. So I would be lost just like the motor control repair tech would be, if working on a bias oscillator circuit for a tape deck. Does that make me less or incomplete as a tech? No, and the same does apply to you in this aspect.
Oh so this is a particularity.
If you have the transformer specifications, I might be able to find one or even get someone like Hammond to add it into their inventory.
There might be one by someone who builds it for a 6bq5 stereo amp somewhere.
LazerBeakShiek
Tell Tony, Eddie an the Cruises IS here
Copied and pasted marshall support emails. yeah help me find the info..Over the years I got about ten guys contacting me if I ever figured it out. They wanted to do it too. go bottom to top-
Hello Mark,
Sorry we do not have this information, it may be worth finding a company that can re-wind the transformer, at least that way they can simply rebuild it with the same guage wire and it’s guaranteed to be correct?
Kind Regards
From: Mark Christesen <mark19741971@att.net>
Sent: 09 November 2020 15:03
To: Joanna Green <joanna.green@marshall.com>
Subject: RE: Transformer for EL-84 20/20 115 volt US
You made it, you should have that information so I can repair my amplifier. Somebody at Marshall should have these specifications. Please direct me to a Marshall head of service.
From: Joanna Green
Sent: Monday, November 9, 2020 6:24 AM
To: Mark Christesen
Subject: RE: Transformer for EL-84 20/20 115 volt US
This is not information that we have on file.
Kind Regards
From: Mark Christesen <mark19741971@att.net>
Sent: 09 November 2020 12:15
To: Joanna Green <joanna.green@marshall.com>
Subject: RE: Transformer for EL-84 20/20 115 volt US
I need voltage & current specs for each winding from Marshall, as those are not made readily apparent on the schematic.
From: Joanna Green
Sent: Monday, November 9, 2020 5:48 AM
To: mark19741971@att.net
Subject: RE: Transformer for EL-84 20/20 115 volt US
Hello Mark,
Thank you for your e-mail regarding your question, the 2020 transformer needs to be replaced for the voltage change, the US transformer is no longer in stock for this model.
Kind Regards
From: Mark Christesen <mark19741971@att.net>
Sent: 07 November 2020 17:22
To: Contact at Marshall.com <contactus@marshall.com>
Subject: Transformer for EL-84 20/20 115 volt US
The EL84 20-20 Power Amp schematic shows a single-primary power transformer for a set mains voltage.
115v: 64591A
230v: 64590A
However, online searching doesn't turn up specs or a ready replacement. For either. What is cross referenceable service replacement?
Hello Mark,
Sorry we do not have this information, it may be worth finding a company that can re-wind the transformer, at least that way they can simply rebuild it with the same guage wire and it’s guaranteed to be correct?
Kind Regards
| ||||||||||
|
Sent: 09 November 2020 15:03
To: Joanna Green <joanna.green@marshall.com>
Subject: RE: Transformer for EL-84 20/20 115 volt US
You made it, you should have that information so I can repair my amplifier. Somebody at Marshall should have these specifications. Please direct me to a Marshall head of service.
From: Joanna Green
Sent: Monday, November 9, 2020 6:24 AM
To: Mark Christesen
Subject: RE: Transformer for EL-84 20/20 115 volt US
This is not information that we have on file.
Kind Regards
| ||||||||||
|
Sent: 09 November 2020 12:15
To: Joanna Green <joanna.green@marshall.com>
Subject: RE: Transformer for EL-84 20/20 115 volt US
I need voltage & current specs for each winding from Marshall, as those are not made readily apparent on the schematic.
From: Joanna Green
Sent: Monday, November 9, 2020 5:48 AM
To: mark19741971@att.net
Subject: RE: Transformer for EL-84 20/20 115 volt US
Hello Mark,
Thank you for your e-mail regarding your question, the 2020 transformer needs to be replaced for the voltage change, the US transformer is no longer in stock for this model.
Kind Regards
| ||||||||||
| ||||||||||
|
Sent: 07 November 2020 17:22
To: Contact at Marshall.com <contactus@marshall.com>
Subject: Transformer for EL-84 20/20 115 volt US
The EL84 20-20 Power Amp schematic shows a single-primary power transformer for a set mains voltage.
115v: 64591A
230v: 64590A
However, online searching doesn't turn up specs or a ready replacement. For either. What is cross referenceable service replacement?
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