I realize that perhaps this could be brought up in a different part of the forum. Please pardon me if this is problematic. I have posted before concerning this project before but can't seem to find where I left off. I purchased what was advertised as a Bogen CHA70 PA amplifier which was actually a CHA75a. I had posted about it before I had received it and some had told me it must be a CHA75a. I thankfully received a link to Electronic Service Manuals.com where I was able to buy a schematic for the correct model for $5. I checked it out and it shows a very "busy" looking circuit with 4 X 6AV5Ga output tubes and some others that I have not seen before. The gentleman who gave me the schematic link told me that it must be a CHA75 and would have 2 X 7027a output tubes!? There have been several others who have also told me that it should have the 7027a's and LO AND BEHOLD, when the amp came, that is exactly what it has! I bought the amp to modify into a guitar amp although it has already had some mods to this end. I wish that I could get a schematic that actually corresponds to what I have, although given time I can get it done.
Does Anybody Have An Idea as to why I have a Sam's PhotoFact schematic that plainly states it is for the CHA75a when the amp in my possession plainly says, right on the front panel the same thing, yet there is such a BIG difference in the circuit.? The tubes on the schematic are as follows: V-1, 12AX7, V-2, 6AU6, V-3, 6U8, V-4,5,6,&7, 6AV5GA, V-8, 6CM7, V-9,10,&11, 5Y3GT.....Now, the amp I actually have has the following, and please pardon the lack of proper numbering of tubes here: 2 X 12AX7, 1 X 6AV6, 1 X 7247, 2 X 7027a, & 3 X GZ34. I realize that this is all probably moot to those here who have much more know how than I do, but I am really curious as to what the deal is, even more than I would like to have the correct diagram! Once again, apologies for the posting improprieties....I Will learn.
Does Anybody Have An Idea as to why I have a Sam's PhotoFact schematic that plainly states it is for the CHA75a when the amp in my possession plainly says, right on the front panel the same thing, yet there is such a BIG difference in the circuit.? The tubes on the schematic are as follows: V-1, 12AX7, V-2, 6AU6, V-3, 6U8, V-4,5,6,&7, 6AV5GA, V-8, 6CM7, V-9,10,&11, 5Y3GT.....Now, the amp I actually have has the following, and please pardon the lack of proper numbering of tubes here: 2 X 12AX7, 1 X 6AV6, 1 X 7247, 2 X 7027a, & 3 X GZ34. I realize that this is all probably moot to those here who have much more know how than I do, but I am really curious as to what the deal is, even more than I would like to have the correct diagram! Once again, apologies for the posting improprieties....I Will learn.
Bogen, and other companies, reserve the rights to make to make changes without notice.
This means they could even substitute a different chassis entirely, if they run out of parts,
or have reliability problems, for example.
Maybe from the tube lineup you can find a Bogen model that corresponds to what you actually have,
and then find a correct schematic.
This means they could even substitute a different chassis entirely, if they run out of parts,
or have reliability problems, for example.
Maybe from the tube lineup you can find a Bogen model that corresponds to what you actually have,
and then find a correct schematic.
You have the "CHA75A" version. It appears the "CHA75" was the first version with the quad of 6AV5GA's. Here's a pic of the "A" version that was listed on reverb.
jeff
If dimwatt has three Mullard GZ34 rectifiers, he can sell them on ebay for a lot of money,
and buy three JJs GZ34s for $30 each to replace them. I would certainly do that.
Reverb sells Mullard GZ34s for around $200 each. I only have one (acquired when new) in my amp.
and buy three JJs GZ34s for $30 each to replace them. I would certainly do that.
Reverb sells Mullard GZ34s for around $200 each. I only have one (acquired when new) in my amp.
Thanks guys for the clarification. That one from Reverb looks pretty much like mine. This one actually looks nicer at a glance. The GZ34's Ain't No Mullard's though. Mine are labeled Bogen. As I understand, there were a few manufacturers who made tubes for Bogen but as is my usual luck, Mullard was not one of them. Also, those 7027a tubes are NOT cheap, although maybe better for a guitar amp than the 6AV5GA tubes. You all would know better than I would about that. I Did realize that Bogen could change chassis/circuits at a whim without notice, just did not expect to run into such a radical change, but that's what I get for expecting things to go smoothly. For what I gave for this amp I could sell the used tubes and make a few bucks. Also, I would imagine the transformers would bring a chunk of change. The whole thing weighs a ton! However, I will "tangle" with this one at some point and perhaps find that elusive schematic in the meantime. Right now, I have two Harmon Kardon Prelude PC-200 amps with a third for parts that I am working on, and I Have proper diagrams for them. They are much simpler projects as near as I can tell. Thanks again for the input everybody, it Is appreciated.
Like any valve amp that is circa 60 years old, it is not uncommon that valves have been replaced, and sometimes even circuitry has been modified.
If you have a 6AV6 in the V2 socket then obviously you need to look at the 6AV6 and 6AU6 datasheets, and then check the V2 related circuitry to see if it was modified for a triode, or the seller just had poor eyesight and put a 6AV6 in there to make the amp look complete. Similar situation for the 7247 in lieu of a 6U8.
Sams circuit is obviously incorrect for your version of 75A. I'd suggest you go back to Electronic Service Manuals.com and tell them the schematic is incorrect for the 75A, or the 75A came in different versions, and see if they have the correct schematic for you, and even ask them to relabel or tag that Sams schematic as suspect so others don't go down the same path.
There is an ampgarage photo on-line of a 75 with 3x GZ34 and a 6AN8 (in lieu of a 6U8), so be aware of what seem to be inconsistancies that you will need to work through.
The 75A obviously has noticeable changes. The change in output stage valves has allowed them to remove the need for a 6CM7 regulator for output stage screen voltage. The 75 schematic with 5Y3GTs indicates they had an issue with supply current exceeding 2x 5Y3, and needed to dovetail in a third 5Y3, and then swap to GZ34's with a higher current rating and lower voltage drop.
I'd strongly recommend you go through your learning curve before trying to alter this amp, as not only do you need to audit the 75A circuit, but a lot of collateral damage could occur without upgrading protection and coming to grips with the circuitry around the 6U8 and what that all means.
If you have a 6AV6 in the V2 socket then obviously you need to look at the 6AV6 and 6AU6 datasheets, and then check the V2 related circuitry to see if it was modified for a triode, or the seller just had poor eyesight and put a 6AV6 in there to make the amp look complete. Similar situation for the 7247 in lieu of a 6U8.
Sams circuit is obviously incorrect for your version of 75A. I'd suggest you go back to Electronic Service Manuals.com and tell them the schematic is incorrect for the 75A, or the 75A came in different versions, and see if they have the correct schematic for you, and even ask them to relabel or tag that Sams schematic as suspect so others don't go down the same path.
There is an ampgarage photo on-line of a 75 with 3x GZ34 and a 6AN8 (in lieu of a 6U8), so be aware of what seem to be inconsistancies that you will need to work through.
The 75A obviously has noticeable changes. The change in output stage valves has allowed them to remove the need for a 6CM7 regulator for output stage screen voltage. The 75 schematic with 5Y3GTs indicates they had an issue with supply current exceeding 2x 5Y3, and needed to dovetail in a third 5Y3, and then swap to GZ34's with a higher current rating and lower voltage drop.
I'd strongly recommend you go through your learning curve before trying to alter this amp, as not only do you need to audit the 75A circuit, but a lot of collateral damage could occur without upgrading protection and coming to grips with the circuitry around the 6U8 and what that all means.
trobbins. Thanks for the very informative reply. I pretty much figured this one would need Considerable further study and as you allude to, a review of my learning curve. This is one reason that I joined this forum. I make notes about the advice given me here even as I continue to "hit the books". The things you have pointed to as being possibilities with this Bogus....I mean Bogen(just kidding), were definitely not on my radar as things to be on the lookout for and I will most assuredly check out. I will also try to contact Electronic Service Manuals.com about this. Hopefully, they will check into it and find out what might be the issue. Anyplace that offers such diagrams for a mere $5 is worth trying to aid. While some of your advice will take me some time to educate myself about, I will do so and I am very thankful to get this level of help, as I don't have any other way except self study from home and experimentation as I also seek the "ultimate tone" from my gear. I will remember your moniker and look for further tidbits of your obvious expertise. Thanks again trobbins!