Hello all.
I am about to become the owner of an ST-500 dual channel stereophonic amplifier made by Dynamic electronics.
the tube configuration consists of:
2x12ax7
1x12au7
4xel84
2xez81
I would like to make this the best little performer I can.
anyone familiar with this amp?
opinions on the output watts? I would be happy at 15-20
upgrades and mods? I am going to budget 300/350 for upgrades.
I have a little background building low watt SE clones in 6v6 configuration for guitar amps. so i'm not totally lost. first time in the audio world.
help and recommendations are greatly appreciated.
I'll post pics when it arrives.
I am about to become the owner of an ST-500 dual channel stereophonic amplifier made by Dynamic electronics.
the tube configuration consists of:
2x12ax7
1x12au7
4xel84
2xez81
I would like to make this the best little performer I can.
anyone familiar with this amp?
opinions on the output watts? I would be happy at 15-20
upgrades and mods? I am going to budget 300/350 for upgrades.
I have a little background building low watt SE clones in 6v6 configuration for guitar amps. so i'm not totally lost. first time in the audio world.
help and recommendations are greatly appreciated.
I'll post pics when it arrives.
I don't find that exact model in the Sams index, but there's a model 100 and ST-100, both in the same folder, and there's a model 500 - perhaps the ST-500 is the same or close.
Some "guesstimates" about the circuitry are possible. 15 WPC is about right, given the almost certain PP 6BQ5/EL84 "finals". The pair of 6CA4/EZ81 rectifiers is a nice, not cheap, touch.
As is, I don't think a mag. phono preamp is present. The 2X 12AX7s would be consumed by that function, leaving no gain for tone controls and power section.
S/F, how radical a "restomod" are you willing to entertain? Improved source selection and new RCA jacks are (IMO) an absolute minimum. Do you want a built in phono preamp? Frankly, the OEM tone controls suck, need to go, and those 'X7s would take on that job well. Switching to SS rectified B+ frees up sockets for signal tubes and also allows for several interesting possibilities.
Full pentode mode "finals" are likely the case. Regulated screen grid B+ is an obvious improvement to that sort of setup.
As is, I don't think a mag. phono preamp is present. The 2X 12AX7s would be consumed by that function, leaving no gain for tone controls and power section.
S/F, how radical a "restomod" are you willing to entertain? Improved source selection and new RCA jacks are (IMO) an absolute minimum. Do you want a built in phono preamp? Frankly, the OEM tone controls suck, need to go, and those 'X7s would take on that job well. Switching to SS rectified B+ frees up sockets for signal tubes and also allows for several interesting possibilities.
Full pentode mode "finals" are likely the case. Regulated screen grid B+ is an obvious improvement to that sort of setup.
Thank you Ei for popping in on my questions.
this amp will see primary duty powering my Klipsch Forte ll's.
an external preamp would be ideal. I listen to vinyl more than anything.
SS rectification is something i'm down with.
I want a tube amp that will make me and my speakers happy. I'm hoping 350.00 in mods will bring it up to snuff. That's not including a tube upgrade.
IMO/IME you should change your view. High PIV Schottky diodes are every bit as quiet as vacuum rectifiers. Do you want good bass? It's no accident that amps like the Harman/Kardon Citation II and McIntosh MC275, which are famous for good bass, have SS rectified B+. "Sagging" B+ is fine in a guitar amp, where the artifact contributes to "tone". OTOH, in a HIFI setup, B+ "sag" sucks.
An external phono preamp definitely makes for a reasonably easy effort.
That sort of budget should be quite adequate.
I'm thinking in terms of no tone controls, as really good tone controls require complexity that can't be supported. While separate, non-ganged, volume controls to set the listening level and channel to channel balance are somewhat fiddly, they are the best way forward for getting good performance, without spending too much. PEC of Canada makes very nice milspec hot molded carbon controls that sound very good and they will last.
A single 12AX7 (shared between channels) and 2X 12BH7s wired to use system 1 in a channel and system 2 in the 2nd channel would fit nicely into an early Fisher (X-100) scheme. Fisher's operating conditions need tweaking, but the topology is fine.
On the subject of O/P tube g2 B+ regulation, read this LENGTHY thread and pay Tom his money.
BTW, that chassis seems reasonably roomy. So adding a choke to obtain a proper CLC B+ filter is a definite consideration. The Triad C-17X is an obvious candidate.
Attachments
Sorry for the misunderstanding.lol
When I said I was "down" with the SS I meant that as a big "yes" to SS.
thanks for posting up the schematic!!
The "iron" in the pics seems substantial. We can try for a bit more power. Between SS rectified B+, "fixed" or combination bias, and Russian 6П14П-EB (6p14p-ev), AKA EL84M, O/P tubes, which are genuine 7189 equivalents, coming reasonably close to the Sherwood S5000 is an achievable goal. 😉
Mouser part # 941-C4D10120D takes care of B+ rectification. Mouser part # 553-C-17X is the filter choke previously mentioned. Photos of the unit's insides, both top and bottom will help in deciding which PSU caps. to buy.
Be sure to bundle your orders with any specific vendor. Otherwise, shipping charges will "eat" too much of your budget.
Mouser part # 941-C4D10120D takes care of B+ rectification. Mouser part # 553-C-17X is the filter choke previously mentioned. Photos of the unit's insides, both top and bottom will help in deciding which PSU caps. to buy.
Be sure to bundle your orders with any specific vendor. Otherwise, shipping charges will "eat" too much of your budget.
Tom, thanks for the OEM schematic.
That setup seems to be another variation on the paraphase splitter theme.
That setup seems to be another variation on the paraphase splitter theme.
Kill the approaching a Sherwood S5000's performance idea. 
A photo in this thread shows what the O/P trafos really look like. 🙁 They are anything but hefty. Phooey!
Realistic possibilities include the Fisher X-100 previously uploaded or a variation on the "El Cheapo" theme I'm associated with.
The Klipsch Forte speakers the amp will be used with will not be impacted by rolling deep bass and infrasonic noise off, as shown in the "El Cheapo" schematic. That roll off protects the O/P "iron" against core saturation, due to a GNFB error correction signal. Given the unfortunate photo, such protection is essential.
A photo in this thread shows what the O/P trafos really look like. 🙁 They are anything but hefty. Phooey!Realistic possibilities include the Fisher X-100 previously uploaded or a variation on the "El Cheapo" theme I'm associated with.
The Klipsch Forte speakers the amp will be used with will not be impacted by rolling deep bass and infrasonic noise off, as shown in the "El Cheapo" schematic. That roll off protects the O/P "iron" against core saturation, due to a GNFB error correction signal. Given the unfortunate photo, such protection is essential.
Attachments
Thanks again for the schematics!
there is WAY too much going on inside this amplifier. I need to simplify things.
my needs are simple. lets start shedding anything deemed unnecessary.
tone controls gone. I'm searching for a suitable preamp.
selector switch. not really needed.
rectifier tubes. going SS
and anything else anyone can see that is not needed.
the O/P iron is going to be replaced with new units. recommendations? my knowledge is limited to Classictone.
praying the P/T is still viable.
The mailman needs to drop this off already.
again, many thanks for the input and shared knowledge.
there is WAY too much going on inside this amplifier. I need to simplify things.
my needs are simple. lets start shedding anything deemed unnecessary.
tone controls gone. I'm searching for a suitable preamp.
selector switch. not really needed.
rectifier tubes. going SS
and anything else anyone can see that is not needed.
the O/P iron is going to be replaced with new units. recommendations? my knowledge is limited to Classictone.
praying the P/T is still viable.
The mailman needs to drop this off already.
again, many thanks for the input and shared knowledge.
Magnetics are costly. You could easily consume the better part of your budget on new O/P trafos. 🙁 For instance, the "gold standard" Dynaclone Z565s cost (sic) $93.95 each. Pretty darned good and definitely more affordable ($63.11 each) are Edcor CXPP25-8Ks.
Frankly, high 90s sensitive Klipsch Forte speakers will easily satisfy Paul Joppa's 102 dB. rule, when fed by the 6 or so WPC a triode wired "El Cheapo" yields. The OEM O/P "iron" in the Dynamic ST-500 will be adequate in that sort of setup. I suspect that damping factor, rather than sheer power, is the issue with Fortes. A triode wired "El Cheapo", with its GNFB loop, will provide a reasonably high damping factor.
A decent Lorlin selector switch costs a few cents more than $5 and it's builder configurable between 2 and 6 sources. 😉 JMO, retain flexibility.
A DIY tubed phono preamp is a project of considerable difficulty. Initially, I suggest you go with a Hagerman "Bugle-2". Being passively equalized and WELL engineered, it will sound good, especially if superior opamps are installed.
Frankly, high 90s sensitive Klipsch Forte speakers will easily satisfy Paul Joppa's 102 dB. rule, when fed by the 6 or so WPC a triode wired "El Cheapo" yields. The OEM O/P "iron" in the Dynamic ST-500 will be adequate in that sort of setup. I suspect that damping factor, rather than sheer power, is the issue with Fortes. A triode wired "El Cheapo", with its GNFB loop, will provide a reasonably high damping factor.
A decent Lorlin selector switch costs a few cents more than $5 and it's builder configurable between 2 and 6 sources. 😉 JMO, retain flexibility.
A DIY tubed phono preamp is a project of considerable difficulty. Initially, I suggest you go with a Hagerman "Bugle-2". Being passively equalized and WELL engineered, it will sound good, especially if superior opamps are installed.
The phase inverter and gain stage are a configuration popular in the '60s - split-load (cathodyne) phase inverter with positive feedback (bootstrapping). This increases gain, and gain is what allows negative feedback to work, so distortion is lowered further. The input sensitivity is high enough at about 400 mV at the 12AX7 input to add a volume pot here and eliminate everything upstream (first gain stage and tone controls). The existing 1 meg volume control is too high, 100K would be better. Any lower and you'll have to make sure your preamp can drive it.
If you have the EZ90s, why not keep them for now - another 30-40V will be harder on the 6BQ5s - they'll likely be beyond ratings if you don't increase the cathode resistors to cool things down.
If you have the EZ90s, why not keep them for now - another 30-40V will be harder on the 6BQ5s - they'll likely be beyond ratings if you don't increase the cathode resistors to cool things down.
Tom,
"El Cheapo" needs about 355 V. of B+. Switching to SS rectification gets us "there".
The 12AT7 splitter/driver works WELL with 200 to 220 V. on the plate and an IB of 3 mA. A 355 V. B+ rail takes care of the decoupling network losses and the drop across the 50 Kohm load resistors.
BTW, the OEM schematic shows 2X EZ81/6CA4s handling B+ rectification. What confuses me is the 140 mA. remark, as a single 6CA4 is good for 150 mA. Perhaps the remark should be 240 mA.
A quartet of 6BQ5/EL84 O/P tubes consume, by themselves, approx. 160 mA. of B+.
"El Cheapo" needs about 355 V. of B+. Switching to SS rectification gets us "there".
The 12AT7 splitter/driver works WELL with 200 to 220 V. on the plate and an IB of 3 mA. A 355 V. B+ rail takes care of the decoupling network losses and the drop across the 50 Kohm load resistors.
BTW, the OEM schematic shows 2X EZ81/6CA4s handling B+ rectification. What confuses me is the 140 mA. remark, as a single 6CA4 is good for 150 mA. Perhaps the remark should be 240 mA.
A quartet of 6BQ5/EL84 O/P tubes consume, by themselves, approx. 160 mA. of B+.Still not here.
I really need the amp in my possession so I can make a decision on what is good and what is bad.
Articles I have read on the Forte ll's claim that they thrive on high current and match well with the Dynaco 70 and 35. Being realistic, I have no expectations of this amp competing in that arena.
But close would be nice.
I really need the amp in my possession so I can make a decision on what is good and what is bad.
Articles I have read on the Forte ll's claim that they thrive on high current and match well with the Dynaco 70 and 35. Being realistic, I have no expectations of this amp competing in that arena.
But close would be nice.
S/F,
The Forte II is rated 99 dB. sensitive. To satisfy Paul Joppa's 102 dB. rule, 2 WPC are more than enough. 4 WPC will yield 106 dB. SPLs. Do you need bleeding ears?
If you rework the unit in "El Cheapo" fashion, the B+ reservoir capacitor will be substantial and reserves of current present. Combine current reserves with a reasonable damping factor and you should do fine.
If you want to try for 15 WPC using full pentode mode "finals", an interesting possibility is employing the Russian 6p15p (preferably the -ev suffixed version). The type contains a fragile screen grid that has to be held at 150 V. That works to your advantage, in that g2 B+ regulation is simple, with a 0A2 per channel getting the job done. FWIW, sockets correctly configured for the 6p15p will accept 6BQ5/EL84s, without incident.
The Forte II is rated 99 dB. sensitive. To satisfy Paul Joppa's 102 dB. rule, 2 WPC are more than enough. 4 WPC will yield 106 dB. SPLs. Do you need bleeding ears?
If you rework the unit in "El Cheapo" fashion, the B+ reservoir capacitor will be substantial and reserves of current present. Combine current reserves with a reasonable damping factor and you should do fine.
If you want to try for 15 WPC using full pentode mode "finals", an interesting possibility is employing the Russian 6p15p (preferably the -ev suffixed version). The type contains a fragile screen grid that has to be held at 150 V. That works to your advantage, in that g2 B+ regulation is simple, with a 0A2 per channel getting the job done. FWIW, sockets correctly configured for the 6p15p will accept 6BQ5/EL84s, without incident.
Do you need bleeding ears?
LOL. No sir. I know that 15wpc is not needed. the amp and speakers will be in a 940 sq ft open space with ten foot ceilings. minimal furnishings. an old 1 room school house. my new play house.
LOL. No sir. I know that 15wpc is not needed. the amp and speakers will be in a 940 sq ft open space with ten foot ceilings. minimal furnishings. an old 1 room school house. my new play house.
epic seller FAIL
Well, this deal is not happening.
It's been over two weeks since contact. it was supposed to be boxed and shipped two weeks ago. I'm just glad no money was sent.
truthfully, if he contacted me tomorrow I would not take it. I lost all faith in the deal and would be uneasy sending money at this point.
So what is a fellow going to do? heart set on tube power but no product.
well, THIS fellow purchased a UA-1 mono block.
and it's time to start a new thread.
This one can die. just like the Dynamic deal.
Well, this deal is not happening.
It's been over two weeks since contact. it was supposed to be boxed and shipped two weeks ago. I'm just glad no money was sent.
truthfully, if he contacted me tomorrow I would not take it. I lost all faith in the deal and would be uneasy sending money at this point.
So what is a fellow going to do? heart set on tube power but no product.
well, THIS fellow purchased a UA-1 mono block.
and it's time to start a new thread.
This one can die. just like the Dynamic deal.
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