FULLY RESTORED PAIR DYNACO MK IV MONOBLOCK TUBE AMPLIFIERS DYNAKIT MarkIV

Description

VINTAGE DYNACO DYNAKIT MK IV MONOBLOCK TUBE POWER AMPLIFIERS
IDENTICALLY MATCHED PAIR - SUPER NICE!
This listing is for the two MK IV amps and tubes only, no other items pictured are included.
Each of these Dynaco MK IV monoblock amplifier produces 40 watts of continuous power with peaks of output power reaching up to 90 watts each.
The benefits and advantages of monoblocks are unparalleled. By having each channel operating completely separate, each output transformer having its own power transformer and circuit board, the stereo separation and complete lack of "sag" due to a shared power source and common rectification, the monoblock is the pinnacle of tube sonics. These are very impressive sounding amplifiers. Essentially the MK IV is 1/2 of a Dynaco 70 with a few extra watts to throw at you. There is a notable difference in the richness and fullness these provide over a Dynaco ST-70. The MK IV is a great sounding amp.
This pair of vintage amps has just undergone a complete electronic and cosmetic restoration. This is a true "built to match pair" of Dynaco MK IV amps. Each amp has been electrically, mechanically and cosmetically restored and then rebuilt to identical standards. The restoration was undertaken focused on making them identically matched and bring them back to a safe and reliable state while improving their their sonic capabilities, overall quality and appearance. They are just a magnificent pair of amps now. I've been breaking these in this week using both a PAS tube preamp and a PAT5 solid state preamp and the amps just sound incredible.
These amps both feature their original vintage cloth lead Dynaco transformers. The PA-135 Power Transformers are mounted on heavy duty rubber isolation bushings to prevent mechanical hum from resonating through the chassis, which might appear as hum or noise through the speakers. Each transformer was tested, cleaned and cosmetically restored. These amplifiers / transformers should be operated only with a voltage reducing device to keep the current at approximately 117 Volts AC. Modern household current, sometimes exceeding 125 volts is simply too high and will shorten the lifespan of the amplifier, causing damage to the transformers and other components. These vintage transformers were designed to operate at 117 volts, not 125 volts. This is important not only for the life and reliability of the transformers and components / tubes, but sonically as well. A variac with a two way adapter plug will work or you can build or buy bucking transformers from several vendors, including a few right here on eBay.
The vintage A470 output transformers are simply legendary. These too both have the cloth covered leads.
The C-354 filter chokes are new from Dynakit. These are encased in a metal housing for improved shielding, quieter operation. The C-354 chokes were repositioned under the chassis to allow for the larger style rubber feet to be installed, thus giving these amps a nice elevated stance. The original location of the filter choke would not allow room to install a foot on that corner of the bottom plate, so the MKIV amps originally sat on 4 small rubber bumpers. The amps look better and are definitely a lot easier to handle with these regular sized feet now in place, there's now room to get your fingers underneath them to pick them up to move them.
Brand new Dynakit PC-4 "B" circuit boards installed on both amps. These use the stock original Dynaco circuit but are made specifically for using 6U8A or 6GH8A tubes instead of the vanishing and expensive 7199 tube. These tubes are nearly identical electrically to each other except the pin out arrangement is different. This board is made so that you do not have to use tube adapters to use the 6U8A / 6GH8A tubes. That's the only difference and the sound is the same as before. Dynaco developed this circuit back when 7199 tube production ceased, thousands of amps came direct from the factory with this circuit, this is not some "hacked together" modification, it's the real deal direct from Dynakit. Contrary to some rumors and audio lore, these tubes sound every bit as good as the 7199. The 6U8A is robust and these amps each include matching military communication grade Raytheon JAN-6U8A (NOS) tubes with their original boxes.
Frequency response is improved by means of using very high quality capacitors and low noise metal film resistors throughout the entire amplifier. I chose Vishay 1839HQ Series capacitors for these amps and they sound absolutely fantastic. I also went with metal film and carbon film resistors in 1% & 5% tolerances to help retain the natural warmth of the original circuit design.
The multi section filter capacitors are new, these are the upgrade version by Dynakit, rated at 40/20/20/20 @ 525V. The bias supply is all new and features 2 watt resistors and dual 100uF 100 Volt capacitors. The bias potentiometers are also brand new, sourced direct from Dynakit. Safety diodes have been added to the rectifier tube sockets under the chassis. This is to prevent component damage from current flashback which can occur during sudden power on/off/on sequences like during a power failure or from anyone accidentally switching the amp off and back on too quickly.
The Ceramic tube sockets have gold contacts. All of the wiring is brand new 20 AWG solid core. The preamp power takeoff socket on the rear of the amp has been wired only for bias set function. Bias is suggested to be set at 1.56 volts DC per amp, the factory specification.
The output terminals are standard screw terminal type and have taps for 4, 8 & 16 Ohms.
The new power switch is a heavy duty C&K sealed type slide switch with a 6 amp 10,000 cycle rating.
The RCA inputs have been updated with very high quality nickel plated jacks, much better than the old original Dynaco jack.
The power cords are new 3 prong type. Each amp includes an inline CL-90 thermistor (inrush current limiter) mounted under the chassis. This device provides a time delay voltage ramp up during power up. The objective is to gradually apply the AC voltage rather than dump a full load on the cold tubes and transformers. It "warms them up" slowly helping to reduce stress and failures from instant power surges that occur when the power switch is activated. The GZ34 rectifier tube also serves a similar purpose, slowly applying power to the amps tubes and audio circuit.
The output tubes are a factory matched quad of EL34's from Nesstone. I have used and sold over 100 amps with these tubes and have simply had no other tube that sounds any better. They are excellent tubes and look great too.
The rectifier tubes are Solid State 5AR4/GZ34 type which also incorporate a thermistor to provide the soft start power up that a vacuum tube GZ34 would provide, both of these copper clad rectifiers are brand new also. The use of the solid state rectifier has several advantages. The B+ sag is reduced resulting in improved bass. Some load reduction for the power transformer is achieved. The socket is a standard 8 pin and any 5AR4 / GZ34 tube can be installed if desired.
The chassis were disassembled to the last screw, hand polished and are really nice. The covers are finished in high heat industrial high heat black and feature new custom made aluminum badges. The covers are both original to each amp but were made at different times, possibly by different manufacturers and the perforations on one amp are slightly larger than those of the other amp. Not a big issue but worth mentioning.
Operationally I would rate these amps a 10 out of 10, they're as good as new, probably better than new. Cosmetically I will rate them at 8.5 out of 10 since there are some marks, some blemishes and typical signs of use and age.
Although fully restored I cannot and do not offer a warranty on these amps since they do use vintage transformers. I do not know the exact production date.
These will each be double boxed and shipped in two seperate boxes, the tubes will be removed for shipping. A tube installation guide is included. The MKIV manual is available for free in many locations on the web and contains full assembly, operating and diagrams with parts lists etc. The manual also explains the bias procedure. These amps have the stock bias circuit and the standard bias procedure in the manual will apply.
Ships UPS Ground only and will be insured for full value - these will be shipped properly
A signature will be required due to the insurance - plan accordingly
once the shipment is in progress no changes to delivery can be made - that's an eBay policy issue, not mine.
no guarantee or warranty due to age
no returns - no refunds
Please see all my other listings for more great Dynaco and Dynakit gear!